fix: FastAPI routing conflict for /placeholders endpoint

Backend:
- Moved /placeholders endpoint BEFORE /{prompt_id} catch-all
- Prevents "placeholders" being parsed as UUID parameter
- Fixes 500 Internal Server Error preventing placeholder loading

Frontend:
- PlaceholderPicker can now load ~120+ system placeholders

Root Cause:
- FastAPI matches routes in order
- Generic /{prompt_id} was catching /placeholders first
- psycopg2 error: invalid input syntax for type uuid: "placeholders"

Version: 0.9p (workflow module)
Part 3: End Node Template Engine

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

backend/routers/prompts.py

@@ -53,6 +53,20 @@ def list_prompts(session: dict=Depends(require_auth)):
         return [r2d(r) for r in cur.fetchall()]
 
 
+@router.get("/placeholders")
+def list_placeholders_endpoint(session: dict=Depends(require_auth)):
+    """
+    Get grouped catalog of available placeholders with descriptions and examples.
+
+    Returns:
+        Dict mapping category to list of {key, description, example}
+
+    IMPORTANT: This endpoint MUST be defined BEFORE /{prompt_id} to avoid routing conflict.
+    """
+    profile_id = session['profile_id']
+    return get_placeholder_catalog(profile_id)
+
+
 @router.get("/{prompt_id}")
 def get_prompt(prompt_id: str, session: dict=Depends(require_auth)):
     """Get single AI prompt by ID (UUID)."""