fix(appview): resolve_profile also looks up DID in profiles cache

A user who set their profile via the PDS push path before
posting anything has a row in `profiles` but no rows in
`posts` — the handle→DID lookup in `resolve_profile` only
checked `posts`, so the route synthesised an empty profile
(`did: ""`) for these users.

The fix: query `profiles` first, fall back to `posts` only
when there's no profile row. The new query uses
`LOWER(handle) = LOWER($1)` against the
`profiles_handle_idx` index (which is therefore no longer
dead weight and stays in 0005; 0007 still drops it idempotently
in case future edits reintroduce the original 'only-by-DID'
pattern).

Verified end-to-end against a running dev stack:
`GET /api/profile/<handle>` now returns the user's profile
metadata even when they have no posts indexed yet.

migrations/appview/0005_profiles.sql

@@ -40,6 +40,7 @@ CREATE TABLE profiles (
     following_count    BIGINT NOT NULL DEFAULT 0,
     indexed_at         TIMESTAMPTZ NOT NULL DEFAULT now()
 );
+CREATE INDEX profiles_handle_idx ON profiles (LOWER(handle));
 CREATE INDEX profiles_indexed_at_idx ON profiles (indexed_at DESC);
 
 -- Backfill: seed a profile row for every handle we've already