feat: Add Postgres Schema, Indexes, and Feed Queries - #4
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Users, follows, and posts sit in 3NF with a composite author timeline index and keyset SQL. PostgresStore implements ActivityStore and runs the shared contract suite against PGlite.
Seed enough posts, ANALYZE, and EXPLAIN the author timeline as an index scan with no sort. Walk each followee through the same index with LATERAL. First page and keyed page are separate statements.
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Postgres now implements ActivityStore. Users, follows, and posts are 3NF tables with a unique handle index, a no-self-follow CHECK, and a composite (author_id, created_at DESC, id DESC) index. Author timelines and the home feed use two statements (first page vs keyed page) so a null cursor cannot force a generic plan. The home feed is CROSS JOIN LATERAL: each followee walks that index newest-first, then the outer query merges.
Same contract suite as the in-memory store. After ANALYZE, EXPLAIN on the author timeline is an Index Scan using posts_author_timeline_idx with no Sort. Tests run the SQL on PGlite. Any { query(sql, params) } client works, including node-postgres.
Closes #3