feat(admin): add user management, quotas, and stats dashboard to admin panel

- Dashboard tab: total users, active users, admins, storage usage, quota warnings
- User management tab: list, edit role, activate/deactivate, update quota, delete
- Self-protection: cannot delete/deactivate/demote yourself
- Paginated user listing (default 50 per page)
- Efficient single-query dashboard stats with direct SQL aggregation
- Quota modal with GB/MB/TB unit selector
- Backend: added methods to UserStoragePort, UserRepository, PgRepository
- Backend: added 7 admin methods to AuthApplicationService
- Backend: added 5 new DTOs for admin operations
- Backend: 7 new endpoints under /api/admin/

Files modified:
- src/application/ports/auth_ports.rs (4 new UserStoragePort methods)
- src/domain/repositories/user_repository.rs (StorageStats + 3 methods)
- src/infrastructure/repositories/pg/user_pg_repository.rs (implementations)
- src/application/services/auth_application_service.rs (admin methods)
- src/application/dtos/settings_dto.rs (5 new DTOs)
- src/interfaces/api/handlers/admin_handler.rs (7 new endpoints)
- static/admin.html (complete UI with 3 tabs: Dashboard, Users, OIDC)

src/application/dtos/settings_dto.rs

@@ -56,3 +56,53 @@ pub struct OidcTestResultDto {
     /// Suggested provider name (derived from issuer hostname)
     pub provider_name_suggestion: Option<String>,
 }
+
+// ============================================================================
+// Admin User Management DTOs
+// ============================================================================
+
+/// Request body for updating a user's role
+#[derive(Debug, Serialize, Deserialize)]
+pub struct UpdateUserRoleDto {
+    pub role: String,
+}
+
+/// Request body for updating a user's active status
+#[derive(Debug, Serialize, Deserialize)]
+pub struct UpdateUserActiveDto {
+    pub active: bool,
+}
+
+/// Request body for updating a user's storage quota
+#[derive(Debug, Serialize, Deserialize)]
+pub struct UpdateUserQuotaDto {
+    /// Quota in bytes. Use 0 for unlimited.
+    pub quota_bytes: i64,
+}
+
+/// Query parameters for listing users
+#[derive(Debug, Serialize, Deserialize)]
+pub struct ListUsersQueryDto {
+    pub limit: Option<i64>,
+    pub offset: Option<i64>,
+}
+
+/// Dashboard statistics
+#[derive(Debug, Serialize, Deserialize)]
+pub struct DashboardStatsDto {
+    // System info
+    pub server_version: String,
+    pub auth_enabled: bool,
+    pub oidc_configured: bool,
+    pub quotas_enabled: bool,
+    // User stats
+    pub total_users: i64,
+    pub active_users: i64,
+    pub admin_users: i64,
+    // Storage stats
+    pub total_quota_bytes: i64,
+    pub total_used_bytes: i64,
+    pub storage_usage_percent: f64,
+    pub users_over_80_percent: i64,
+    pub users_over_quota: i64,
+}