perf(issue#6): migrate ShareFsRepository to PostgreSQL

- Add storage.shares table with indexes on token, (item_id, item_type), created_by
- Create SharePgRepository with indexed SQL queries and window-function pagination
- Rewire DI to inject SharePgRepository with PgPool instead of config
- Delete legacy share_fs_repository.rs (295 lines of JSON file I/O)
- Remove dead module declaration from repositories/mod.rs

Eliminates O(n) full-file JSON reads/writes, TOCTOU races, and crash
corruption risk. All share operations now use indexed PG queries.

src/infrastructure/repositories/pg/mod.rs

@@ -8,6 +8,7 @@ mod favorites_pg_repository;
 mod recent_items_pg_repository;
 mod session_pg_repository;
 mod settings_pg_repository;
+mod share_pg_repository;
 mod transaction_utils;
 mod user_pg_repository;
 
@@ -30,5 +31,6 @@ pub use folder_db_repository::FolderDbRepository;
 pub use recent_items_pg_repository::RecentItemsPgRepository;
 pub use session_pg_repository::SessionPgRepository;
 pub use settings_pg_repository::SettingsPgRepository;
+pub use share_pg_repository::SharePgRepository;
 pub use trash_db_repository::TrashDbRepository;
 pub use user_pg_repository::UserPgRepository;