feat: AI tag suggestion endpoint

[meninoebom]

Phase 4a — Backend

Add a backend endpoint that uses Claude to suggest tags for a theme based on its content.

Acceptance Criteria

• anthropic added to pyproject.toml dependencies
• ANTHROPIC_API_KEY env var documented and added to Railway
• POST /api/themes/{theme_id}/suggest-tags endpoint (auth required)
  • Fetches theme body_md from DB
  • Queries all existing tag names for context (prefer reuse)
  • Calls Claude Haiku with prompt to suggest 3–5 tags
  • Returns {"tags": ["tag-one", "tag-two"]}, normalized via existing tag format
• Returns 404 if theme not found
• Tests with mocked anthropic client

Notes

• No migration needed
• Tag names normalized through existing [a-z0-9-] format

[meninoebom]

Shipped in PR #69. POST /api/themes/{id}/suggest-tags — calls Claude Haiku with theme body + existing tag context, returns 3–5 normalized slug tags. Logic extracted to app/tag_suggester.py for clean testability. 5 tests cover happy path, normalization, 404, and 503 fallback.