fix: Add missing GET /api/prompts/{id} endpoint

Critical Backend Bug:
- Frontend calls api.getPrompt(id) → GET /api/prompts/{uuid}
- Backend had NO endpoint for single prompt retrieval by ID
- Result: 405 Method Not Allowed

Backend Endpoints Before:
✓ GET /api/prompts - List all
✓ POST /api/prompts - Create
✓ PUT /api/prompts/{id} - Update
✗ GET /api/prompts/{id} - MISSING!

Backend Endpoints After:
✓ GET /api/prompts - List all
✓ GET /api/prompts/{id} - Get single (NEW)
✓ POST /api/prompts - Create
✓ PUT /api/prompts/{id} - Update

Implementation:
- Added get_prompt(prompt_id: str) function
- Returns single prompt by UUID
- 404 if not found
- Requires auth (admin or user)

This fixes:
- Workflow loading after save (loadWorkflow calls getPrompt)
- Workflow editing from admin list (Edit button calls getPrompt)
- All 405 Method Not Allowed errors

Root Cause: Backend was incomplete, missing basic CRUD read-by-id endpoint

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("/{prompt_id}")
+def get_prompt(prompt_id: str, session: dict=Depends(require_auth)):
+    """Get single AI prompt by ID (UUID)."""
+    with get_db() as conn:
+        cur = get_cursor(conn)
+        cur.execute("SELECT * FROM ai_prompts WHERE id=%s", (prompt_id,))
+        row = cur.fetchone()
+
+        if not row:
+            raise HTTPException(status_code=404, detail="Prompt not found")
+
+        return r2d(row)
+
+
 @router.post("")
 def create_prompt(p: PromptCreate, session: dict=Depends(require_admin)):
     """Create new AI prompt (admin only)."""