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/db.py

@@ -813,6 +813,10 @@ def init_db():
         conn.execute("CREATE INDEX IF NOT EXISTS idx_agent_conv_llm ON agent_conversations(provider, model)")
         conn.commit()
 
+        # ── v5.2.0: apply any pending VERSIONED migrations (schema_version) ──
+        from app.migrations import apply_migrations
+        apply_migrations(conn)
+
 
 @contextmanager
 def get_conn():