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/migrations.py

@@ -0,0 +1,172 @@
+"""FlowDeck — versioned schema migrations (lightweight, no Alembic).
+
+This replaces the previous "ad-hoc" approach where every new schema change was
+appended directly to `app/db.py::init_db()` with no tracking. A `schema_version`
+table now records the highest applied migration; the full baseline schema
+(created idempotently by `init_db`) is treated as version 1, and any incremental
+change is expressed as an ordered, versioned step below and applied exactly once.
+
+Each migration function receives a raw ``sqlite3.Connection`` (WAL + foreign keys
+already enabled) and must be written idempotently (``IF NOT EXISTS`` / guarded
+``ALTER TABLE``) so it is safe even if partially re-run.
+"""
+from __future__ import annotations
+
+import logging
+import sqlite3
+from typing import Callable, List, Tuple
+
+logger = logging.getLogger(__name__)
+
+# The full baseline schema created by `app.db::init_db()` is "version 1".
+BASELINE_VERSION = 1
+
+# (version, name, apply_fn). Kept sorted by version at registration time.
+MIGRATIONS: List[Tuple[int, str, Callable[[sqlite3.Connection], None]]] = []
+
+
+def register(version: int, name: str) -> Callable:
+    """Decorator registering a migration in the ordered registry."""
+    if any(v == version for v, _, _ in MIGRATIONS):
+        raise ValueError(f"Duplicate migration version {version}")
+
+    def decorator(fn: Callable[[sqlite3.Connection], None]):
+        MIGRATIONS.append((version, name, fn))
+        MIGRATIONS.sort(key=lambda item: item[0])
+        return fn
+
+    return decorator
+
+
+def _ensure_table(conn: sqlite3.Connection) -> None:
+    conn.execute(
+        """
+        CREATE TABLE IF NOT EXISTS schema_version (
+            version INTEGER PRIMARY KEY,
+            name TEXT NOT NULL,
+            applied_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+        )
+        """
+    )
+
+
+def current_version(conn: sqlite3.Connection) -> int:
+    _ensure_table(conn)
+    row = conn.execute(
+        "SELECT COALESCE(MAX(version), 0) AS v FROM schema_version"
+    ).fetchone()
+    return int(row[0])
+
+
+def fts5_available() -> bool:
+    """True when the bundled SQLite ships the FTS5 extension."""
+    probe = sqlite3.connect(":memory:")
+    try:
+        probe.execute("CREATE VIRTUAL TABLE _fts5_probe USING fts5(x)")
+        return True
+    except sqlite3.OperationalError:
+        return False
+    finally:
+        probe.close()
+
+
+def apply_migrations(conn: sqlite3.Connection) -> int:
+    """Seal the baseline schema (version 1) and apply pending migrations.
+
+    Returns the resulting schema version.
+    """
+    _ensure_table(conn)
+    applied = current_version(conn)
+
+    if applied < BASELINE_VERSION:
+        # The pre-existing schema (already created by init_db) is our baseline.
+        conn.execute(
+            "INSERT OR IGNORE INTO schema_version (version, name) VALUES (?, ?)",
+            (BASELINE_VERSION, "baseline"),
+        )
+        conn.commit()
+        applied = BASELINE_VERSION
+
+    for version, name, fn in MIGRATIONS:
+        if version <= applied:
+            continue
+        fn(conn)
+        conn.execute(
+            "INSERT INTO schema_version (version, name) VALUES (?, ?)",
+            (version, name),
+        )
+        conn.commit()
+        applied = version
+        logger.info("Applied migration %d: %s", version, name)
+
+    return applied
+
+
+# ═══════════════════════════════════════════════════════════════════════════
+# Migrations
+# ═══════════════════════════════════════════════════════════════════════════
+
+
+@register(2, "missing indexes")
+def _migration_missing_indexes(conn: sqlite3.Connection) -> None:
+    """Add the indexes flagged in the roadmap (fast lookups by email, forge user)."""
+    for ddl in (
+        "CREATE INDEX IF NOT EXISTS idx_users_email ON users(email)",
+        "CREATE INDEX IF NOT EXISTS idx_user_oauth_tokens_user ON user_oauth_tokens(user_id, provider)",
+        "CREATE INDEX IF NOT EXISTS idx_collections_workspace ON collections(workspace_id)",
+        "CREATE INDEX IF NOT EXISTS idx_pages_workspace ON pages(workspace_id)",
+        "CREATE INDEX IF NOT EXISTS idx_pages_deleted ON pages(deleted_at)",
+    ):
+        conn.execute(ddl)
+
+
+@register(3, "full-text search (FTS5)")
+def _migration_fts5(conn: sqlite3.Connection) -> None:
+    """Create a full-text index over pages (title + content) for the command palette.
+
+    Kept in sync via row-level triggers on the ``pages`` table so page
+    insert/update/delete are reflected immediately. Skips gracefully if the
+    bundled SQLite lacks FTS5 (search then falls back to LIKE).
+    """
+    if not fts5_available():
+        logger.warning("FTS5 unavailable — skipping full-text index (LIKE fallback active)")
+        return
+
+    conn.execute("CREATE VIRTUAL TABLE IF NOT EXISTS pages_fts USING fts5(title, body)")
+
+    conn.execute(
+        """
+        CREATE TRIGGER IF NOT EXISTS pages_fts_ai AFTER INSERT ON pages BEGIN
+            INSERT INTO pages_fts(rowid, title, body)
+            VALUES (new.id, COALESCE(new.title, ''), COALESCE(new.content, ''));
+        END
+        """
+    )
+    # `pages_fts` is a standalone FTS5 table (it stores its own content), so deletes
+    # use a plain DELETE by rowid (NOT the special 'delete' insert that only applies
+    # to external-content/contentless FTS5 tables).
+    conn.execute(
+        """
+        CREATE TRIGGER IF NOT EXISTS pages_fts_ad AFTER DELETE ON pages BEGIN
+            DELETE FROM pages_fts WHERE rowid = old.id;
+        END
+        """
+    )
+    conn.execute(
+        """
+        CREATE TRIGGER IF NOT EXISTS pages_fts_au AFTER UPDATE ON pages BEGIN
+            DELETE FROM pages_fts WHERE rowid = old.id;
+            INSERT INTO pages_fts(rowid, title, body)
+            VALUES (new.id, COALESCE(new.title, ''), COALESCE(new.content, ''));
+        END
+        """
+    )
+
+    # Backfill the index from any rows that already exist.
+    conn.execute(
+        """
+        INSERT INTO pages_fts(rowid, title, body)
+        SELECT id, COALESCE(title, ''), COALESCE(content, '') FROM pages
+        WHERE deleted_at IS NULL
+        """
+    )