perf(thumbnail): cache attached-blob lookups on the request path

Every thumbnail request paid an uncached storage.file_attached_blobs
point query before it could answer — including 304 revalidations and
RAM thumbnail hits, where the ETag path (thumbnail_content_id) probes
the row every time and tier 2b probes it again with the same key. A
photos grid revalidating 60 thumbnails per visit meant 60+ point
queries per browse, repeated on every visit.

find_attached_blob now reads through a process-local moka cache in
DedupService, keyed by the row's (file_id, kind, variant) PK, holding
positive and negative entries (most files have no attached preview, so
the negative side carries the win). Two rules keep it honest:

- DB faults are surfaced as Err and never cached — a transient outage
  cannot freeze "no attached blob" into a negative entry (a read
  failure is never proof that data is absent). The public signature is
  unchanged; the SQL body moved to find_attached_blob_uncached.
- Writes invalidate eagerly: store_attached_blob and the Inserted arm
  of store_attached_blob_if_absent on success, and deletions via
  ThumbnailRefreshHook::on_file_deleted, which all three production
  delete paths (single file, folder cascade, trash clear) fire after
  the DELETE commits. The 60s TTL bounds only what the process cannot
  see (bare SQL, copy_file_satellites races).

The Nextcloud preview endpoint rides the same lookup and benefits
identically. Five in-memory contract tests pin the cache behaviour,
including the fault-not-cached rule.

Co-Authored-By: Claude Code <noreply@anthropic.com>

src/infrastructure/services/thumbnail_service.rs

@@ -1874,10 +1874,16 @@ impl crate::application::ports::file_lifecycle::FileLifecycleHook for ThumbnailR
     fn on_file_deleted(&self, file_id: &str) {
         let thumbnail = self.thumbnail.clone();
         let file_id = file_id.to_string();
+        // The row is gone (CASCADE cleared file_attached_blobs) — drop any
+        // cached attached-blob lookup for this file too. TTL would bound the
+        // staleness anyway, but deletes are rare and the cache lookup after a
+        // delete is pure waste.
+        let dedup = self.dedup.clone();
         tokio::spawn(async move {
             if let Err(e) = thumbnail.delete_thumbnails(&file_id).await {
                 tracing::warn!("Failed to delete thumbnails for file {}: {}", file_id, e);
             }
+            dedup.invalidate_attached_blobs_for_file(&file_id).await;
         });
     }
 }