fix: Migration-Fehler - meas_id Spalte in ai_insights

PROBLEM:
- Backend crasht beim Start auf Prod
- Migration schlägt fehl: column 'meas_id' does not exist
- SQLite ai_insights hat Legacy-Spalte meas_id
- PostgreSQL schema hat diese Spalte nicht mehr

FIX:
- COLUMN_WHITELIST für ai_insights hinzugefügt
- Nur erlaubte Spalten werden migriert:
  id, profile_id, scope, content, created
- meas_id wird beim Import gefiltert

DATEIEN:
- backend/migrate_to_postgres.py

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

backend/migrate_to_postgres.py

@@ -69,6 +69,12 @@ BOOLEAN_COLUMNS = {
     'ai_prompts': ['active'],
 }
 
+# Column whitelist for tables that have schema differences
+# Only these columns will be migrated (filters out legacy columns)
+COLUMN_WHITELIST = {
+    'ai_insights': ['id', 'profile_id', 'scope', 'content', 'created'],
+}
+
 
 # ================================================================
 # CONVERSION HELPERS
@@ -173,8 +179,17 @@ def migrate_table(pg_conn, table: str) -> Dict[str, int]:
     # Convert rows
     converted_rows = [convert_row(row, table) for row in sqlite_rows]
 
-    # Get column names
-    columns = list(converted_rows[0].keys())
+    # Get column names - filter if whitelist exists for this table
+    if table in COLUMN_WHITELIST:
+        allowed_columns = COLUMN_WHITELIST[table]
+        # Filter rows to only include allowed columns
+        converted_rows = [
+            {k: v for k, v in row.items() if k in allowed_columns}
+            for row in converted_rows
+        ]
+        columns = allowed_columns
+    else:
+        columns = list(converted_rows[0].keys())
     cols_str = ', '.join(columns)
     placeholders = ', '.join(['%s'] * len(columns))