feat: thumbnail dedup — store thumbnails by blob_hash instead of file_id (#233)

Thumbnails are now keyed by blob_hash on disk so identical files share
a single set of thumbnails (icon/preview/large). For 4000 duplicate
files with the same content, this reduces thumbnail storage from 12,000
files to just 3.

Changes:
- get_thumbnail_path() keys by blob_hash instead of file_id
- get_thumbnail(), get_cached_thumbnail(), generate_all_sizes_background()
  accept blob_hash parameter for disk dedup
- generate_all_sizes_background() fast path: if blob-hash thumbnails
  already exist on disk, skip image processing entirely and just
  populate moka cache for the new file_id
- delete_thumbnails() only invalidates moka cache (shared disk
  thumbnails must not be deleted when one file is removed)
- delete_blob_thumbnails() added for GC; garbage_collect() now cleans
  up orphaned thumbnail files alongside blob files
- External thumbnails (video frames) stored as ext-{file_id}.jpg
  since they are client-generated and not dedup-able
- ThumbnailPort trait updated with blob_hash parameters
- All handler call sites updated (file_handler, preview_handler)
- Tests updated for new signatures

src/application/ports/thumbnail_ports.rs

@@ -68,18 +68,27 @@ pub trait ThumbnailPort: Send + Sync + 'static {
 
     /// Get a thumbnail, generating it on-demand if needed.
     ///
-    /// Returns the thumbnail bytes in WebP format.
+    /// `blob_hash` is the content hash used as the disk storage key
+    /// (dedup: identical blobs share one set of thumbnails).
     async fn get_thumbnail(
         &self,
         file_id: &str,
+        blob_hash: &str,
         size: ThumbnailSize,
         original_path: &Path,
     ) -> Result<Bytes, DomainError>;
 
     /// Generate all thumbnail sizes for a file in the background.
     ///
-    /// Called after file upload to pre-generate thumbnails.
-    fn generate_all_sizes_background(self: Arc<Self>, file_id: String, original_path: PathBuf);
+    /// `blob_hash` is the content hash used as the disk storage key.
+    /// If thumbnails already exist for this hash, only the moka cache
+    /// is populated (zero CPU for image processing).
+    fn generate_all_sizes_background(
+        self: Arc<Self>,
+        file_id: String,
+        blob_hash: String,
+        original_path: PathBuf,
+    );
 
     /// Delete all thumbnails for a file.
     async fn delete_thumbnails(&self, file_id: &str) -> Result<(), DomainError>;
@@ -87,9 +96,14 @@ pub trait ThumbnailPort: Send + Sync + 'static {
     /// Try to get a cached thumbnail without generating one.
     ///
     /// Returns `None` if no cached thumbnail exists on disk or in memory.
-    /// Used for non-image file types (videos) where thumbnails are
-    /// generated client-side and uploaded.
-    async fn get_cached_thumbnail(&self, file_id: &str, size: ThumbnailSize) -> Option<Bytes>;
+    /// `blob_hash` is used to locate the file on disk. If `None`, only
+    /// the in-memory moka cache is checked.
+    async fn get_cached_thumbnail(
+        &self,
+        file_id: &str,
+        blob_hash: Option<&str>,
+        size: ThumbnailSize,
+    ) -> Option<Bytes>;
 
     /// Store an externally-generated thumbnail (e.g. client-side video frame).
     ///