Adds GET /community/posts, GET /community/posts/{slug}, GET /community/feed.rss,
GET /community/local-feed, POST /community/posts, DELETE /community/posts/{slug},
POST /community/posts/{slug}/fork, and POST /community/posts/{slug}/fork-adapt (501 stub).
Wires init_community_store into main.py lifespan. 7 new tests; 115 total passing.
