feat: prevent duplicate rest day types per date (Migration 012)

Problem: User can create multiple rest days of same type per date
(e.g., 2x Mental Rest on 2026-03-23) - makes no sense.

Solution: UNIQUE constraint on (profile_id, date, focus)

## Migration 012:
- Add focus column (extracted from rest_config JSONB)
- Populate from existing data
- Add NOT NULL constraint
- Add CHECK constraint (valid focus values)
- Add UNIQUE constraint (profile_id, date, focus)
- Add index for performance

## Backend:
- Insert focus column alongside rest_config
- Handle UniqueViolation gracefully
- User-friendly error: "Du hast bereits einen Ruhetag 'Muskelregeneration' für 23.03."

## Benefits:
- DB-level enforcement (clean)
- Fast queries (no JSONB scan)
- Clear error messages
- Prevents: 2x muscle_recovery same day
- Allows: muscle_recovery + mental_rest same day ✓

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

backend/migrations/012_rest_days_unique_focus.sql

@@ -0,0 +1,34 @@
+-- Migration 012: Unique constraint on (profile_id, date, focus)
+-- v9d Phase 2a: Prevent duplicate rest day types per date
+-- Date: 2026-03-22
+
+-- Add focus column (extracted from rest_config for performance + constraints)
+ALTER TABLE rest_days
+ADD COLUMN IF NOT EXISTS focus VARCHAR(20);
+
+-- Populate from existing JSONB data
+UPDATE rest_days
+SET focus = rest_config->>'focus'
+WHERE focus IS NULL;
+
+-- Make NOT NULL (safe because we just populated all rows)
+ALTER TABLE rest_days
+ALTER COLUMN focus SET NOT NULL;
+
+-- Add CHECK constraint for valid focus values
+ALTER TABLE rest_days
+ADD CONSTRAINT valid_focus CHECK (
+    focus IN ('muscle_recovery', 'cardio_recovery', 'mental_rest', 'deload', 'injury')
+);
+
+-- Add UNIQUE constraint: Same profile + date + focus = duplicate
+ALTER TABLE rest_days
+ADD CONSTRAINT unique_rest_day_per_focus
+UNIQUE (profile_id, date, focus);
+
+-- Add index for efficient queries by focus
+CREATE INDEX IF NOT EXISTS idx_rest_days_focus
+ON rest_days(focus);
+
+-- Comment for documentation
+COMMENT ON COLUMN rest_days.focus IS 'Extracted from rest_config.focus for performance and constraints. Prevents duplicate rest day types per date.';

backend/routers/rest_days.py

@@ -10,6 +10,7 @@ from datetime import datetime, timedelta
 from fastapi import APIRouter, HTTPException, Depends, Header
 from pydantic import BaseModel, Field
 from psycopg2.extras import Json
+from psycopg2.errors import UniqueViolation
 
 from db import get_db, get_cursor, r2d
 from auth import require_auth
@@ -89,23 +90,39 @@ def create_rest_day(
 
     # Convert RestConfig to dict for JSONB storage
     config_dict = data.rest_config.model_dump()
+    focus = data.rest_config.focus
 
-    with get_db() as conn:
-        cur = get_cursor(conn)
+    try:
+        with get_db() as conn:
+            cur = get_cursor(conn)
 
-        # Insert (multiple entries per date allowed)
-        cur.execute(
-            """
-            INSERT INTO rest_days (profile_id, date, rest_config, note, created_at)
-            VALUES (%s, %s, %s, %s, CURRENT_TIMESTAMP)
-            RETURNING id, profile_id, date, rest_config, note, created_at
-            """,
-            (pid, data.date, Json(config_dict), data.note)
+            # Insert (multiple entries per date allowed, but not same focus)
+            cur.execute(
+                """
+                INSERT INTO rest_days (profile_id, date, focus, rest_config, note, created_at)
+                VALUES (%s, %s, %s, %s, %s, CURRENT_TIMESTAMP)
+                RETURNING id, profile_id, date, focus, rest_config, note, created_at
+                """,
+                (pid, data.date, focus, Json(config_dict), data.note)
+            )
+
+            result = cur.fetchone()
+            return r2d(result)
+    except UniqueViolation:
+        # User-friendly error for duplicate focus
+        focus_labels = {
+            'muscle_recovery': 'Muskelregeneration',
+            'cardio_recovery': 'Cardio-Erholung',
+            'mental_rest': 'Mentale Erholung',
+            'deload': 'Deload',
+            'injury': 'Verletzungspause',
+        }
+        focus_label = focus_labels.get(focus, focus)
+        raise HTTPException(
+            400,
+            f"Du hast bereits einen Ruhetag '{focus_label}' für {data.date}. Bitte wähle einen anderen Typ oder lösche den bestehenden Eintrag."
         )
 
-        result = cur.fetchone()
-        return r2d(result)
-
 
 @router.get("/{rest_day_id}")
 def get_rest_day(
@@ -159,6 +176,9 @@ def update_rest_day(
     if data.rest_config:
         updates.append("rest_config = %s")
         values.append(Json(data.rest_config.model_dump()))
+        # Also update focus column if config changed
+        updates.append("focus = %s")
+        values.append(data.rest_config.focus)
 
     if data.note is not None:
         updates.append("note = %s")