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/domain/src/catalog/ports.rs

@@ -35,6 +35,10 @@ pub trait AssetRepository: Send + Sync {
 #[async_trait]
 pub trait AssetMetadataRepository: Send + Sync {
     async fn find_by_asset(&self, asset_id: &SystemId) -> Result<Vec<AssetMetadata>, DomainError>;
+    async fn find_by_assets(
+        &self,
+        asset_ids: &[SystemId],
+    ) -> Result<Vec<AssetMetadata>, DomainError>;
     async fn find_by_asset_and_source(
         &self,
         asset_id: &SystemId,
@@ -78,7 +82,11 @@ pub trait DerivativeRepository: Send + Sync {
 #[async_trait]
 pub trait DuplicateRepository: Send + Sync {
     async fn find_by_id(&self, id: &SystemId) -> Result<Option<DuplicateGroup>, DomainError>;
-    async fn find_unresolved(&self) -> Result<Vec<DuplicateGroup>, DomainError>;
+    async fn find_unresolved(
+        &self,
+        limit: u32,
+        offset: u32,
+    ) -> Result<Vec<DuplicateGroup>, DomainError>;
     async fn find_by_asset(&self, asset_id: &SystemId) -> Result<Vec<DuplicateGroup>, DomainError>;
     async fn save(&self, group: &DuplicateGroup) -> Result<(), DomainError>;
 }