feat(core): v5.0.0 command palette + FTS5 search, v5.2.0 versioned migrations

- Command palette Ctrl+K/Ctrl+P (base.html): universal search, fuzzy
  highlight, keyboard nav, quick actions. openQuickFind now opens it.
- GET /api/search endpoint + search service: unified pages + databases
  search, FTS5 with LIKE fallback, trash excluded, user-scoped.
- app/migrations.py: lightweight versioned migration runner (schema_version
  table); baseline schema = v1, new steps applied in order.
- Migration v2: missing indexes (users.email, user_oauth_tokens, etc).
- Migration v3: FTS5 pages_fts + sync triggers + backfill.
- VERSION + app.main -> 5.0.0; CHANGELOG + ROADMAP updated.
- +8 tests (tests/test_search_migrations.py). 259 passed (3 pre-existing
  PDF/weasyprint env failures unrelated).

app/routers/search.py

@@ -0,0 +1,27 @@
+"""FlowDeck — unified search router (v5.0.0).
+
+``GET /api/search?q=`` backs the Ctrl+K command palette. Returns matching
+editor pages and databases scoped to the current user's accessible workspaces.
+"""
+from __future__ import annotations
+
+from fastapi import APIRouter, Request, Query
+
+from app.auth.session import SessionManager
+from app.services.search import search as search_service
+
+router = APIRouter(tags=["search"])
+
+
+@router.get("/api/search")
+async def search(request: Request, q: str = Query(default="")):
+    user = SessionManager.decode_session(request.cookies.get("flowdeck_session", ""))
+    user_id = user.get("id") if user and user.get("id") else None
+
+    data = search_service(q, user_id=user_id)
+    return {
+        "query": q,
+        "pages": data["pages"],
+        "collections": data["collections"],
+        "total": len(data["pages"]) + len(data["collections"]),
+    }