test(backend): DB-backed tests for the risky SQL; test isolation; clippy cleanup

All 40 backend tests were pure-function tests — not one line of SQL ran under `cargo test`,
even though the riskiest code in the repo is SQL. Add 12 DB-backed `#[sqlx::test]` tests
(fresh throwaway DB per test, real migrations) pinning the invariants that code is
load-bearing for, each mutation-verified to fail on broken code:

  export_epoch.rs (8): epoch is post-increment on release (a worker born pre-increment is
    inert); epoch strictly monotonic across reopen; a retired-epoch worker's writes are
    no-ops; export_current is EXACTLY the invariant (8-case table); the ViewerOnly
    carry-forward carries a done ZIP forward AND matches nothing when the ZIP is unfinished
    (the af997a8 strand bug); boot recovery doesn't clobber a live ZIP.
  upload_concurrency.rs (4): the atomic quota UPDATE stops two stale-snapshot uploads from
    both committing (sequential AND concurrent-tx via EPQ); the FOR SHARE upload lock
    serializes against release, blocking a photo from committing after the export snapshot.

Deleting FOR SHARE, or the carry-forward's `status='done'`, each makes a test fail.

Test isolation: TRUNCATE now also resets disk_cache and sse_tickets. The stale disk
reading was harmless only while quotas were globally off in e2e (they no longer are — see
the new quota spec), i.e. two holes were masking each other.

clippy: `cargo clippy --all-targets -- -D warnings` now passes (it did not). Deleted the
dead jobs.rs (an unused BackgroundJob sketch) and unused model methods; `#[allow(dead_code)]`
+ comment on the sqlx FromRow field-sets that ARE populated by the DB. No SQL or logic
changed. `cargo test` requires DATABASE_URL by design.

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

backend/src/error.rs

@@ -75,10 +75,10 @@ impl IntoResponse for AppError {
         }
 
         let mut resp = (status, axum::Json(body)).into_response();
-        if let Some(secs) = retry_after_secs {
-            if let Ok(val) = axum::http::HeaderValue::from_str(&secs.to_string()) {
-                resp.headers_mut().insert(axum::http::header::RETRY_AFTER, val);
-            }
+        if let Some(secs) = retry_after_secs
+            && let Ok(val) = axum::http::HeaderValue::from_str(&secs.to_string())
+        {
+            resp.headers_mut().insert(axum::http::header::RETRY_AFTER, val);
         }
         resp
     }