Add DateModified to domain models + delta API endpoints

[fboucher]

Parent PRD

#110

Depends on

Nothing — fully server-side, can be worked in parallel.

Summary

Add DateModified (UTC timestamp) to the Post and Note domain models and their Azure Table Storage representations. Extend existing API endpoints to support a modifiedAfter query parameter for delta sync.

Domain changes

• Add DateModified (DateTime UTC) to Post (in NoteBookmark.Domain/Post.cs)
• Add DateModified (DateTime UTC) to Note (in NoteBookmark.Domain/Note.cs)
• Set DateModified automatically on every create and update operation

API changes

• GET /api/posts?modifiedAfter={ISO8601} — returns only posts modified after the given timestamp; returns all posts if param omitted (non-breaking)
• GET /api/notes?modifiedAfter={ISO8601} — same for notes
• PATCH /api/posts/{id} and PATCH /api/notes/{id} correctly update DateModified

Tests

• GET /posts?modifiedAfter={ts} returns only records modified after ts
• GET /notes?modifiedAfter={ts} same
• Both endpoints return all records when param omitted
• Empty-result and multi-result cases covered
• Use existing WebApplicationFactory + NoteBookmarkApiTestFactory pattern

Acceptance Criteria

• DateModified field on Post and Note
• All create/update paths set DateModified = DateTime.UtcNow
• modifiedAfter param on both list endpoints (non-breaking — omitting returns all)
• API integration tests pass

Sync stories from PRD

User stories 7, 8, 19