fix: startup crash - init_db() jetzt mit Error-Handling

PROBLEM:
- Backend crasht beim Start auf Prod
- Login zeigt HTML statt JSON (Backend nicht erreichbar)
- Ursache: init_db() wirft Exception beim Startup

FIX:
1. startup_event() wrapped in try-except (non-fatal)
2. init_db() prüft ob ai_prompts Tabelle existiert
3. init_db() hat eigenen try-except
4. Bessere Fehlermeldungen in stdout

ERGEBNIS:
- Backend startet auch wenn init_db() fehlschlägt
- Pipeline-Prompt kann manuell angelegt werden falls nötig
- Login funktioniert wieder

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

backend/main.py

@@ -43,7 +43,11 @@ AVATAR_COLORS = ['#1D9E75','#378ADD','#D85A30','#EF9F27','#7F77DD','#D4537E','#6
 @app.on_event("startup")
 async def startup_event():
     """Run migrations and initialization on startup."""
+    try:
         init_db()
+    except Exception as e:
+        print(f"⚠️  init_db() failed (non-fatal): {e}")
+        # Don't crash on startup - pipeline prompt can be created manually
 
 def init_db():
     """Initialize database - Schema is loaded by startup.sh"""
@@ -51,8 +55,21 @@ def init_db():
     # This function kept for backwards compatibility
 
     # Ensure "pipeline" master prompt exists
+    try:
         with get_db() as conn:
             cur = get_cursor(conn)
+            # Check if table exists first
+            cur.execute("""
+                SELECT EXISTS (
+                    SELECT FROM information_schema.tables
+                    WHERE table_schema = 'public'
+                    AND table_name = 'ai_prompts'
+                ) as table_exists
+            """)
+            if not cur.fetchone()['table_exists']:
+                print("⚠️  ai_prompts table doesn't exist yet - skipping pipeline prompt creation")
+                return
+
             cur.execute("SELECT COUNT(*) as count FROM ai_prompts WHERE slug='pipeline'")
             if cur.fetchone()['count'] == 0:
                 cur.execute("""
@@ -68,6 +85,9 @@ def init_db():
                 """)
                 conn.commit()
                 print("✓ Pipeline master prompt created")
+    except Exception as e:
+        print(f"⚠️  Could not create pipeline prompt: {e}")
+        # Don't fail startup - prompt can be created manually
 
 # ── Helper: get profile_id from header ───────────────────────────────────────
 def get_pid(x_profile_id: Optional[str] = Header(default=None)) -> str: