perf(quota): stop recomputing storage usage on every GET /api/auth/me

GET /api/auth/me ran a synchronous O(N) SUM(size) over all the user's
files plus an unconditional UPDATE of auth.users on every call — one of
the most frequently hit endpoints — adding per-request latency, DB write
load, dead tuples and WAL even when nothing changed.

- /api/auth/me now serves the cached storage_used_bytes column instead of
  recomputing it inline.
- New StorageUsageService::start_reconciliation_job runs a periodic sweep
  on the maintenance pool that keeps the cached value current for every
  mutation (uploads, deletes, trash), so freshness no longer depends on
  hitting /me. Interval via OXICLOUD_STORAGE_USAGE_RECONCILE_SECS (default
  600s, floored at 30s; first sweep deferred one interval to avoid boot load).
- update_storage_usage only writes when the value actually changes
  (IS DISTINCT FROM), so the sweep produces no dead tuple / WAL on no-ops.
- New covering partial index idx_files_user_size_active makes the usage
  SUM an index-only scan instead of a heap scan over all the user's files.

Also collapse the same pre-existing clippy collapsible_else_if in
carddav_handler that blocks the -D warnings gate on this base.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/infrastructure/repositories/pg/user_pg_repository.rs

@@ -411,7 +411,11 @@ impl UserRepository for UserPgRepository {
         Ok(user)
     }
 
-    /// Updates only the storage usage of a user
+    /// Updates only the storage usage of a user.
+    ///
+    /// The `IS DISTINCT FROM` guard makes this a no-op when the value is
+    /// unchanged — which is the common case for the periodic reconciliation
+    /// sweep — so it produces no dead tuple and no WAL when nothing changed.
     async fn update_storage_usage(
         &self,
         user_id: Uuid,
@@ -420,10 +424,10 @@ impl UserRepository for UserPgRepository {
         sqlx::query(
             r#"
             UPDATE auth.users
-            SET 
+            SET
                 storage_used_bytes = $2,
                 updated_at = NOW()
-            WHERE id = $1
+            WHERE id = $1 AND storage_used_bytes IS DISTINCT FROM $2
             "#,
         )
         .bind(user_id)