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/storage/Cargo.toml

@@ -16,6 +16,7 @@ tracing     = { workspace = true }
 bytes       = { workspace = true }
 futures     = { workspace = true }
 tokio       = { workspace = true, features = ["fs"] }
+tokio-util  = { workspace = true }
 object_store = { version = "0.11" }
 
 [dev-dependencies]