fix(thumbnails): two defects the copy test exposed

1. Per-file overrides must beat the content tier in RAM as well as on
   disk. The content-keyed lookup ran first, so a thumbnail already
   rendered from the file's content sat in RAM under content(blob_hash)
   and shadowed a preview uploaded afterwards — permanently. Invisible
   before the moka rekey, because both lived under one file-id key and
   the upload simply overwrote the render. Order is now uniform:
   per-file RAM, per-file disk (ext-), per-file DB, then content RAM,
   blob-hash disk, derived blob.

2. store_attached_blob never wrote a row. Its RETURNING clause compared
   the stored hash against EXCLUDED, and PostgreSQL only permits
   EXCLUDED in the SET and WHERE of DO UPDATE — a runtime syntax error
   on every call. The superseded hash now comes from a SELECT taken
   before the upsert; losing that race leaves one stale reference, which
   the manifest recompute reports, rather than anything being lost.

The second hid behind the first for a whole cycle, and behind
`ext-{file_id}.jpg`: the ORIGINAL kept serving its uploaded preview from
local disk, so the feature looked healthy. Only a copy, which has a
different file_id and therefore no ext- file, depends on the row — and
the row was never there. The handler's best-effort warn! completed the
disguise, so it is now error!: a failure there means copies silently
lose the preview, and nothing else signals it.

src/interfaces/api/handlers/file_handler.rs

@@ -768,11 +768,15 @@ impl FileHandler {
             )
             .await
         {
-            tracing::warn!(
+            // ERROR, not WARN: the sidecar keeps the feature looking healthy
+            // on this box, so nothing else signals that copies are silently
+            // losing the preview. A syntax error in the upsert hid behind a
+            // warning for an entire test cycle exactly this way.
+            tracing::error!(
                 target: "oxicloud::dedup",
                 error = %e,
                 file_id = %id,
-                "failed to record attached thumbnail; sidecar written, copies will not inherit it"
+                "failed to record attached thumbnail; sidecar written, copies will NOT inherit it"
             );
         }