feat(at-identity): PdsHandleResolver for cluster-local DID→handle resolution

The AppView's handle_sync worker consulted the public PLC directory
and the did:web: HTTPS resolver only. DIDs hosted on the local PDS
(notably did🔑 users and any other operator-hosted method)
weren't reachable without an external round trip, and unresolvable
DIDs (did🔑 not on this PDS, did:foo: anything) blocked the
100-row batch forever because did🔑 sorts lexicographically
before did:plc: / did:web:.

This commit adds:

* `PdsHandleResolver` (at-identity) — POSTs the DID as the
  `handle` field to the PDS's resolveHandle XRPC method. The PDS
  now recognises a `did:` prefix and does a PK lookup on
  `users.did`, returning `{did, handle}`. The resolver reads
  the `handle` field, so the AppView finally gets a real local
  handle for did🔑 users without ever dialing plc.directory.
* A 2 s timeout per request (was 10 s) and `DISPATCH_CONCURRENCY =
  8` so the worker caps a 100-DID batch at ~2 s with parallel
  dispatch instead of the ~17 min worst case the old serial + 10 s
  setup allowed.
* A new `posts.handle_sync_attempted_at` column (migration 0006)
  and `mark_attempted()` helper. The SELECT filter excludes rows
  attempted within the last hour, so an unresolvable DID dominates
  at most one batch before the worker advances. Cleared on success.
* `PDS_INTERNAL_URL` config so the AppView can reach the PDS via
  a cluster-internal hostname when the public URL isn't routable
  from inside the cluster.

Tests:
* `crates/at-identity/src/pds_handle.rs` — 4 unit tests against a
  stub HTTP server (200/404/5xx/missing-did-field).
* Existing handle_sync integration tests updated to wire in the
  new `pds_resolver` field.

crates/appview/Cargo.toml

@@ -37,6 +37,7 @@ at-identity = { workspace = true }
 uuid = { workspace = true }
 rustls = { version = "0.23", default-features = false, features = ["ring", "std", "logging", "tls12"] }
 base64 = { workspace = true }
+futures = { workspace = true }
 
 [dev-dependencies]
 tokio = { workspace = true }