perf: scale fixes for 1M+ photo libraries

Indexes: share_targets.target_id, duplicate_groups.status,
GIN on stacks members + duplicate candidates JSONB,
composite (owner_user_id, created_at DESC) on assets.

N+1 elimination: batch metadata loading via find_by_assets(ids)
using WHERE asset_id = ANY($1), used in timeline + sidecar export.

Visibility: cache find_targets_for_user per request via OnceCell,
extract filter_visible helper to reduce duplication.

Streaming: FileStoragePort.open_file() returns (DataStream, u64),
LocalFileStorage uses ReaderStream instead of loading full file.
serve_file/serve_derivative use Body::from_stream().

Unbounded queries: sidecar full_export/import batched in 500-row
chunks instead of u32::MAX. find_unresolved paginated with
limit/offset. list_duplicates API accepts pagination params.

crates/adapters/postgres/migrations/015_performance_indexes.sql

@@ -0,0 +1,5 @@
+CREATE INDEX idx_share_targets_target ON share_targets(target_id);
+CREATE INDEX idx_duplicate_groups_status ON duplicate_groups(status);
+CREATE INDEX idx_stacks_members ON asset_stacks USING GIN (members);
+CREATE INDEX idx_duplicate_candidates ON duplicate_groups USING GIN (candidates);
+CREATE INDEX idx_assets_created ON assets(owner_user_id, created_at DESC);

crates/adapters/postgres/src/catalog/mod.rs

@@ -336,6 +336,23 @@ impl AssetMetadataRepository for PostgresAssetMetadataRepository {
         Ok(rows.into_iter().map(Into::into).collect())
     }
 
+    async fn find_by_assets(
+        &self,
+        asset_ids: &[SystemId],
+    ) -> Result<Vec<AssetMetadata>, DomainError> {
+        let uuids: Vec<Uuid> = asset_ids.iter().map(|id| *id.as_uuid()).collect();
+        let rows = sqlx::query_as::<_, AssetMetadataRow>(
+            "SELECT asset_id, metadata_source, data, updated_at
+             FROM asset_metadata WHERE asset_id = ANY($1)",
+        )
+        .bind(&uuids)
+        .fetch_all(&self.pool)
+        .await
+        .map_pg()?;
+
+        Ok(rows.into_iter().map(Into::into).collect())
+    }
+
     async fn find_by_asset_and_source(
         &self,
         asset_id: &SystemId,
@@ -482,11 +499,18 @@ impl DuplicateRepository for PostgresDuplicateRepository {
         Ok(row.map(Into::into))
     }
 
-    async fn find_unresolved(&self) -> Result<Vec<DuplicateGroup>, DomainError> {
+    async fn find_unresolved(
+        &self,
+        limit: u32,
+        offset: u32,
+    ) -> Result<Vec<DuplicateGroup>, DomainError> {
         let rows = sqlx::query_as::<_, GroupRow>(
             "SELECT group_id, detection_method, status, candidates
-             FROM duplicate_groups WHERE status = 'unresolved'",
+             FROM duplicate_groups WHERE status = 'unresolved'
+             ORDER BY group_id LIMIT $1 OFFSET $2",
         )
+        .bind(limit as i64)
+        .bind(offset as i64)
         .fetch_all(&self.pool)
         .await
         .map_pg()?;