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/common/di.rs

@@ -37,7 +37,7 @@ use crate::domain::services::i18n_service::I18nService;
 use crate::infrastructure::repositories::pg::{
     FileBlobReadRepository, FileBlobWriteRepository, FolderDbRepository, TrashDbRepository,
 };
-use crate::infrastructure::repositories::share_fs_repository::ShareFsRepository;
+use crate::infrastructure::repositories::pg::SharePgRepository;
 use crate::infrastructure::services::file_content_cache::{
     FileContentCache, FileContentCacheConfig,
 };
@@ -320,13 +320,14 @@ impl AppServiceFactory {
     pub fn create_share_service(
         &self,
         repos: &RepositoryServices,
+        db_pool: &Arc<PgPool>,
     ) -> Option<Arc<dyn crate::application::ports::share_ports::ShareUseCase>> {
         if !self.config.features.enable_file_sharing {
             tracing::info!("File sharing service is disabled in configuration");
             return None;
         }
 
-        let share_repository = Arc::new(ShareFsRepository::new(Arc::new(self.config.clone())));
+        let share_repository = Arc::new(SharePgRepository::new(db_pool.clone()));
 
         // Build a password hasher for share password verification
         let password_hasher: Arc<dyn crate::application::ports::auth_ports::PasswordHasherPort> =
@@ -435,7 +436,7 @@ impl AppServiceFactory {
         let mut apps = self.create_application_services(&core, &repos, trash_service.clone());
 
         // 5. Share service
-        let share_service = self.create_share_service(&repos);
+        let share_service = self.create_share_service(&repos, &pool);
         apps.share_service = share_service.clone();
 
         // 6. Database-dependent services (PgPool always available in blob model)