feat: content-driven features — full-text search, previews, export/download

[Alimedhat000]

Context

Per the #47 resolution, Document.content is a read-only mirror of Yjs state, maintained exclusively by Hocuspocus sync (server/src/lib/dbPersistence.ts). REST never writes it. This issue tracks the consumer features that justify keeping the mirror (Option A).

Features

1. Full-text document search — search documents by content and title.

   • Candidate: Postgres tsvector generated column on Document.content + GIN index; weight title higher than content.
   • Endpoint: GET /document/search?q=... scoped to docs the user owns/collaborates on.

2. Document previews — content snippets in dashboard cards / list views (first N lines), sourced from the mirror without touching Yjs state.

3. Export & download — download a document as:

   • Markdown (.md) — direct from content
   • PDF — render markdown → HTML → PDF server-side or client-side print pipeline

Constraints

• All features are read-only consumers of content; none may introduce a new writer (that's how Document.content has two competing writers: Yjs persistence vs REST autosave #47 happened).
• If write-path needs ever appear (e.g., import), they must go through Yjs seeding (YjsDocumentState), not direct content writes.