fix: add missing display_name to PluginModel creation in lifespan

The PluginModel.display_name column is NOT NULL but was not set when
auto-installing builtin plugins during app startup, causing IntegrityError.
Fix mirrors the pattern used in registry.install().

app/main.py

@@ -120,6 +120,7 @@ async def lifespan(app: FastAPI):
                 # Create DB record for this builtin plugin
                 plugin_record = PluginModel(
                     name=name,
+                    display_name=plugin.manifest.display_name,
                     version=plugin.manifest.version,
                     status="installed",
                     active=True,
@@ -186,7 +187,7 @@ def create_app() -> FastAPI:
     app.include_router(ai_copilot.router)
     app.include_router(workflows.router)
 
-    # ─── Register plugin routes (before SPA catch-all) ──────────────────
+    # ── Register plugin routes (before SPA catch-all) ────────────────
     registry = get_registry()
     try:
         registry.discover_builtins()
@@ -205,7 +206,7 @@ def create_app() -> FastAPI:
     except Exception as exc:
         logger.warning(f"Plugin discovery failed: {exc}")
 
-    # ─── Serve frontend static files (SPA) ───────────────────────────────
+    # ── Serve frontend static files (SPA) ────────────────────────────
     # Mount built frontend assets (JS, CSS, images)
     frontend_dist = os.path.join(os.path.dirname(__file__), "..", "frontend", "dist")
     if os.path.isdir(frontend_dist):