perf: round 23 — Postgres query-shape pass: typed JSONB decode, drive-policy borrow-deserialize, user-profile join!, subject-group CTE reuse, dedup unzip

Benchmark-gated, same rule as ROUND2-22: BEFORE/AFTER with a value-equivalence
gate and rollback-on-regression. Two harnesses — bench_round23_micro (no
Postgres; deterministic allocation gate) and bench_round23_queries (live
Postgres; p50 latency + strict equivalence gate against seeded fixtures). See
benches/ROUND23.md.

- J1: contact_pg_repository::row_to_contact (+ the inlined contact_group sibling)
  decode the 3 JSONB columns via sqlx::types::Json<T> (one from_slice pass)
  instead of row.get::<serde_json::Value> + from_value (a throwaway Value DOM
  per column, walked a second time). Per contact row of every list / multiget /
  CardDAV sync. Micro 84 -> 33 allocs/op (2.15x); PG 3794 -> 2360 ns/contact
  (1.61x) on 500 real rows.
- J2: DrivePolicies::from_value deserializes straight from the borrow
  (T::deserialize(&Value)) instead of from_value(value.clone()) — dropping the
  full-DOM clone on every drive-policy read (move/copy, share, grant); one-line
  body change, all 7 callers unchanged. Micro 5 -> 0 allocs/op (11.51x).
- P1: get_user_profile overlaps the two independent caller+target reads with
  tokio::join! (self-case still a single fetch; caller-error precedence
  preserved via caller_res? first) instead of two serial round-trips. PG
  577 -> 312 us/call (1.85x).
- G1: subject_group remove_member computes the child's transitive-user recursive
  CTE once and reuses it for both the would-empty pre-check and the cache
  invalidation, instead of running the identical CTE twice (the edge delete is
  above the child, so its descendants can't change). PG 829 -> 412 us/removal
  (2.01x).
- U1: dedup_service (store_loose_chunks final registration + the ingest
  run_rollback) reshapes the owned, dead-after Vec<(String,i64)> via
  into_iter().unzip() instead of cloning every 64-byte hash for the
  sync_blobs(&[String]) + UNNEST bind. Micro 256 -> 0 hash clones.

Verified: cargo clippy --features bench --all-targets -D warnings clean, cargo
fmt --all --check clean, cargo test --lib --features bench = 529 passed / 0
failed. The PG benches run against a local PostgreSQL 16 (schema applied from
migrations/); every equivalence gate passes.

The download_zip per-item N+1 (the audit's highest raw-latency candidate) is
deferred to a dedicated pass: its fix moves the sole authorization inside the
stream call, so it needs an AuthZ-ordering + anti-enumeration proof, not a perf
banner.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DKyQ4AnYtgp1JtjzweyMeo

src/application/services/auth_application_service.rs

@@ -1932,17 +1932,28 @@ impl AuthApplicationService {
         expose_system_users: bool,
         pool: &sqlx::PgPool,
     ) -> Result<UserDto, DomainError> {
-        let caller = self.user_storage.get_user_by_id(caller_id).await?;
-
-        // (1) Self.
+        // (1) Self — a single fetch suffices (the check compares the input
+        // UUIDs, so the target read is never needed on this path).
         if caller_id == target_id {
+            let caller = self.user_storage.get_user_by_id(caller_id).await?;
             return Ok(UserDto::from(caller));
         }
 
+        // Caller and target are independent point reads (the self-case already
+        // returned; the branch above compares input UUIDs, not fetched data) —
+        // overlap them with `join!` instead of two serial round-trips.
+        // `caller_res?` first preserves the caller-error precedence of the old
+        // sequential form. (benches/ROUND23.md §P1)
+        let (caller_res, target_res) = tokio::join!(
+            self.user_storage.get_user_by_id(caller_id),
+            self.user_storage.get_user_by_id(target_id)
+        );
+        let caller = caller_res?;
+
         // Anti-enumeration: NotFound for everything that doesn't pass.
         // Convert a real NotFound on `target` to the same anonymous 404,
         // so existence isn't leaked through differential responses.
-        let target = match self.user_storage.get_user_by_id(target_id).await {
+        let target = match target_res {
             Ok(u) => u,
             Err(e) if e.kind == ErrorKind::NotFound => {
                 tracing::info!(