fix(downloads): floor max_concurrent + stable share order (R12 S5 review)

- Floor the effective max_concurrent at 1 on the read path (quota.resolve)
  and in the claim SQL (GREATEST(..., 1)). admin_set_quota already floors on
  write and the sysconfig default's min is 1, but a stray stored 0 (direct DB
  edit) would diverge the two "at limit" checks — the SQL claim-window
  exclusion can't see a user "capped" with zero running jobs, while
  at_concurrency_limit can — reopening the starvation the fairness fix closes.
  Keeping the effective limit >= 1 makes the two checks provably agree.
- shared_with_me now orders by (created_at DESC, id DESC) to match
  list_downloads, so the newly-added 500 cap has a stable boundary among
  same-timestamp shares instead of a non-deterministic one.
- 0062 upgrade loop: rename the unused loop var "table" to "_table"
  (matching the underscore-marked "_col").

Test: a stored max_concurrent=0 flooder no longer wedges the claim loop.

backend/tests/test_download_limits.py

@@ -80,6 +80,30 @@ def test_all_candidates_capped_returns_none(db):
     assert _claim_job() is None
 
 
+def test_stored_zero_concurrency_is_floored_not_starving(db):
+    # A stray max_concurrent=0 is only reachable by a direct DB edit (admin_set_quota floors at 1),
+    # but it must not wedge the claim loop. The effective limit floors to 1 on BOTH the read path
+    # (resolve -> at_concurrency_limit) and in the claim SQL (GREATEST(..., 1)), so the two "at
+    # limit" checks agree: the user's own first job is claimable and its flood never monopolizes
+    # the window with permanently-ineligible jobs. (Before the floor, _claim_job returned None here
+    # — the guard rejected all 60 of A's jobs — and any user behind them starved.)
+    a = User(email="zero@example.com")
+    b = User(email="behind@example.com")
+    db.add_all([a, b])
+    db.flush()
+    _quota(db, a.id, max_concurrent=0)  # nonsensical stored value
+    _quota(db, b.id, max_concurrent=2)
+    for i in range(60):
+        _job(db, a.id, queue_pos=i)
+    _job(db, b.id, queue_pos=100)
+    db.commit()
+
+    claimed = _claim_job()
+    assert claimed is not None  # the loop is not wedged
+    db.expire_all()
+    assert db.get(DownloadJob, claimed).user_id == a.id  # A's own first job runs (floored to 1)
+
+
 def test_unlimited_user_is_never_excluded(db):
     u = User(email="unlimited@example.com")
     db.add(u)