feat(appview): profile cache + Jetstream indexing + denormalised counts

Adds the AppView-side half of the profile feature so non-local-PDS
authors also get their profile metadata indexed (the Jetstream
identity event stream only carries the handle, not display name /
bio / avatar). The PDS-push path was already wired by the previous
commit; this lands the Jetstream path.

Migration 0005:

* `profiles` table keyed by DID with display_name / description /
  avatar_cid / banner_cid plus denormalised post_count /
  follower_count / following_count. Backfilled from the posts
  table on apply.
* `posts.avatar_cid` column — populated from the profiles cache
  at `upsert_post` time so the PostCard can render an avatar
  inline without a per-row PDS round trip.

Migration 0007 (clean-up): the original 0005 also created a
`LOWER(handle)` index that no query uses; this drops it
idempotently so dev DBs that already applied 0005 converge.

Indexer (`crates/appview/src/indexer.rs`):

* New `app.bsky.actor.profile` arm in `apply_commit` calls
  `upsert_profile` on create, DELETEs the row on delete. Handle
  is looked up from `posts` (the Jetstream commit envelope
  doesn't carry it).
* `upsert_post` signature is now `&mut PostRow` so it can fill
  `row.avatar_cid` from the profiles cache; the ON CONFLICT
  clause uses `COALESCE(EXCLUDED, posts)` so re-indexing doesn't
  overwrite an already-known avatar.
* `upsert_profile` writes display_name / description /
  avatar_cid / banner_cid + the denormalised counts.
* `blob_link_of` helper accepts both `{ $type, ref.$link }`
  and legacy flat `{ $link }` blob-ref shapes.

Ingest (`crates/appview/src/ingest.rs`):

* `app.bsky.actor.profile` create/delete arms in the PDS-push
  path. The handle-fallback previously did `SELECT handle FROM
  users WHERE did = $1` — but the AppView has no `users` table
  (it's PDS-owned state). Replaced with a simple use-what-the-PDS-
  sent approach; the handle_sync worker fills the column later.

Routes (`crates/appview/src/routes.rs`):

* `resolve_profile` reads the denormalised profile fields from
  the cache. When no profile row exists the `post_count` fallback
  uses a live `SELECT COUNT(*)` instead of `posts.len()`, so
  prolific authors without a profile row report the real count
  rather than the 50-post slice cap.

Tests (DB-gated, run when DATABASE_URL_APPVIEW is set):

* `blob_link_of_modern_shape` / `_legacy_flat_link` /
  `_missing_field`.
* `upsert_profile_round_trip` — insert + replace semantics.
* `apply_commit_indexes_profile_create` — end-to-end Jetstream
  arm + delete.

migrations/appview/0007_drop_unused_profile_handle_idx.sql

@@ -0,0 +1,12 @@
+-- AppView database schema 0007: drop unused `profiles_handle_idx`.
+--
+-- The original 0005 migration created a `LOWER(handle)` index on
+-- `profiles`, anticipating handle-based lookups. In practice every
+-- caller derives a DID first (via `posts.handle` or the handle-sync
+-- worker) and then queries `profiles` by PK — so the index is dead
+-- weight in storage and write-amplification cost.
+--
+-- This migration drops it idempotently (`IF EXISTS`) so dev DBs that
+-- already applied 0005 also converge. New installs no longer create
+-- the index (0005 was edited when this was discovered).
+DROP INDEX IF EXISTS profiles_handle_idx;