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.

Cargo.toml

@@ -32,6 +32,7 @@ email_address = "0.2"
 sha2          = "0.10"
 uuid          = { version = "1.0", features = ["v4", "serde"] }
 chrono        = { version = "0.4", features = ["serde"] }
+tokio-util    = { version = "0.7", features = ["io"] }
 dotenvy       = "0.15"
 tracing       = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter", "fmt"] }