Implements a complete feed pipeline: extract user interests from stored
memories (via mem0), derive hashtags, fetch posts from Mastodon public
timelines, score posts by relevance, and display ranked results.

Backend: feed models (PostSource, Post, Interest), InterestExtractor
(two-pass aggregation of categories + entities with weighted scoring),
PostFetcher (concurrent Mastodon hashtag timeline fetching), PostScorer
(interest matching + recency decay), FeedService orchestrator, and
REST router with 9 endpoints under /api/feed.

Frontend: FeedPage with interest filter chips, ranked post cards,
source management modal, pagination, and bookmark/seen actions.
Hooks use React Query with dual Keycloak/legacy auth support.

Gated behind social_feed feature flag (disabled by default).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>