feat: Auto-adjust start_date to first available measurement

**User Feedback:** "Macht es nicht Sinn, den nächsten verfügbaren Wert
am oder nach dem Startdatum automatisch zu ermitteln und auch das
Startdatum dann automatisch auf den Wert zu setzen?"

**New Logic:**
1. User sets start_date: 2026-01-01
2. System finds FIRST measurement >= 2026-01-01 (e.g., 2026-01-15: 88 kg)
3. System auto-adjusts:
   - start_date → 2026-01-15
   - start_value → 88 kg
4. User sees: "Start: 88 kg (15.01.26)" ✓

**Benefits:**
- User doesn't need to know exact date of first measurement
- More user-friendly UX
- Automatically finds closest available data

**Implementation:**
- Changed query from "BETWEEN date ±7 days" to "WHERE date >= target_date"
- Returns dict with {'value': float, 'date': date}
- Both create_goal() and update_goal() now adjust start_date automatically

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/routers/goals.py

@@ -395,11 +395,16 @@ def create_goal(data: GoalCreate, session: dict = Depends(require_auth)):
             start_value = data.start_value
         elif start_date < date.today():
             # Historical start date - try to get historical value
-            start_value = _get_historical_value_for_goal_type(conn, pid, data.goal_type, start_date)
-            if start_value is None:
-                # No data on that date, fall back to current value
+            historical_data = _get_historical_value_for_goal_type(conn, pid, data.goal_type, start_date)
+            if historical_data is not None:
+                # Use the actual measurement date and value
+                start_date = historical_data['date']
+                start_value = historical_data['value']
+                print(f"[INFO] Auto-adjusted start_date to {start_date} (first measurement)")
+            else:
+                # No data found, fall back to current value and keep original date
                 start_value = current_value
-                print(f"[WARN] No historical data for {data.goal_type} on {start_date}, using current value")
+                print(f"[WARN] No historical data for {data.goal_type} on or after {start_date}, using current value")
         else:
             # Start date is today, use current value
             start_value = current_value
@@ -514,15 +519,23 @@ def update_goal(goal_id: str, data: GoalUpdate, session: dict = Depends(require_
                 goal_row = cur.fetchone()
                 if goal_row:
                     goal_type = goal_row['goal_type']
-                    print(f"[DEBUG] Looking up historical value for {goal_type} on {data.start_date}")
-                    historical_value = _get_historical_value_for_goal_type(conn, pid, goal_type, data.start_date)
-                    print(f"[DEBUG] Historical value result: {historical_value}")
-                    if historical_value is not None:
+                    print(f"[DEBUG] Looking up historical value for {goal_type} on or after {data.start_date}")
+                    historical_data = _get_historical_value_for_goal_type(conn, pid, goal_type, data.start_date)
+                    print(f"[DEBUG] Historical data result: {historical_data}")
+                    if historical_data is not None:
+                        # Update both start_date and start_value with actual measurement
+                        actual_date = historical_data['date']
+                        actual_value = historical_data['value']
+
+                        # Replace the start_date in updates with the actual measurement date
+                        updates[-1] = "start_date = %s"  # Update the last added start_date
+                        params[-1] = actual_date
+
                         updates.append("start_value = %s")
-                        params.append(historical_value)
-                        print(f"[INFO] Auto-populated start_value from {data.start_date}: {historical_value}")
+                        params.append(actual_value)
+                        print(f"[INFO] Auto-adjusted to first measurement: {actual_date} = {actual_value}")
                     else:
-                        print(f"[WARN] No historical data found for {goal_type} around {data.start_date}")
+                        print(f"[WARN] No historical data found for {goal_type} on or after {data.start_date}")
                 else:
                     print(f"[ERROR] Could not find goal with id {goal_id}")
 
@@ -683,19 +696,19 @@ def _get_current_value_for_goal_type(conn, profile_id: str, goal_type: str) -> O
     # Delegate to universal fetcher (Phase 1.5)
     return get_current_value_for_goal(conn, profile_id, goal_type)
 
-def _get_historical_value_for_goal_type(conn, profile_id: str, goal_type: str, target_date: date) -> Optional[float]:
+def _get_historical_value_for_goal_type(conn, profile_id: str, goal_type: str, target_date: date) -> Optional[dict]:
     """
-    Get historical value for a goal type on a specific date.
-    Looks for closest value within ±7 days window.
+    Get historical value for a goal type on or after a specific date.
+    Finds the FIRST available measurement >= target_date.
 
     Args:
         conn: Database connection
         profile_id: User's profile ID
         goal_type: Goal type key (e.g., 'weight', 'body_fat')
-        target_date: Date to query (can be historical)
+        target_date: Desired start date (will find first measurement on or after this date)
 
     Returns:
-        Historical value or None if not found
+        Dict with {'value': float, 'date': date} or None if not found
     """
     from goal_utils import get_goal_type_config, get_cursor
 
@@ -726,20 +739,16 @@ def _get_historical_value_for_goal_type(conn, profile_id: str, goal_type: str, t
         else:
             date_col = 'date'
 
+        # Find first measurement on or after target_date
         query = f"""
-            SELECT {source_column}
+            SELECT {source_column}, {date_col} as measurement_date
             FROM {source_table}
             WHERE profile_id = %s
-              AND {date_col} BETWEEN %s AND %s
-            ORDER BY ABS({date_col} - %s::date)
+              AND {date_col} >= %s
+            ORDER BY {date_col} ASC
             LIMIT 1
         """
-        params = (
-            profile_id,
-            target_date - timedelta(days=7),
-            target_date + timedelta(days=7),
-            target_date
-        )
+        params = (profile_id, target_date)
 
         print(f"[DEBUG] Query: {query}")
         print(f"[DEBUG] Params: {params}")
@@ -751,12 +760,24 @@ def _get_historical_value_for_goal_type(conn, profile_id: str, goal_type: str, t
 
         if row:
             value = row[source_column]
+            measurement_date = row['measurement_date']
+
             # Convert Decimal to float
-            result = float(value) if value is not None else None
-            print(f"[DEBUG] Returning value: {result}")
+            result_value = float(value) if value is not None else None
+
+            # Handle different date types (date vs datetime)
+            if hasattr(measurement_date, 'date'):
+                # It's a datetime, extract date
+                result_date = measurement_date.date()
+            else:
+                # It's already a date
+                result_date = measurement_date
+
+            result = {'value': result_value, 'date': result_date}
+            print(f"[DEBUG] Returning: {result}")
             return result
 
-        print(f"[DEBUG] No data found in date range")
+        print(f"[DEBUG] No data found on or after {target_date}")
         return None
     except Exception as e:
         print(f"[ERROR] Failed to get historical value for {goal_type} on {target_date}: {e}")