perf(feed): replace correlated subqueries with LEFT JOIN aggregations

Feed queries ran 5 correlated subqueries per row (3 COUNT + 2 EXISTS
for engagement counts and viewer context). Replaced with LEFT JOIN
aggregations computed once per query. Adds migration 016 with indexes
on likes(thought_id), boosts(thought_id), thoughts(in_reply_to_id),
and compound viewer-context indexes — expected to drop ~3s queries to
<100ms on typical page sizes.

Also removes WebFinger from the footer (requires query params, zero
standalone value as a link).

crates/adapters/postgres/migrations/016_engagement_indexes.sql

@@ -0,0 +1,10 @@
+-- Indexes for feed engagement counts and sorting.
+-- likes and boosts are joined/counted per thought on every feed query.
+-- thoughts(in_reply_to_id) is scanned for reply_count.
+CREATE INDEX IF NOT EXISTS idx_likes_thought_id         ON likes(thought_id);
+CREATE INDEX IF NOT EXISTS idx_boosts_thought_id        ON boosts(thought_id);
+CREATE INDEX IF NOT EXISTS idx_thoughts_in_reply_to_id  ON thoughts(in_reply_to_id) WHERE in_reply_to_id IS NOT NULL;
+
+-- Viewer-context lookups: "did I like/boost this?"
+CREATE INDEX IF NOT EXISTS idx_likes_user_thought        ON likes(user_id, thought_id);
+CREATE INDEX IF NOT EXISTS idx_boosts_user_thought       ON boosts(user_id, thought_id);