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/app/downloads/quota.py

@@ -43,10 +43,16 @@ class QuotaExceeded(Exception):
 def resolve(db: Session, user_id: int) -> QuotaLimits:
     row = db.get(DownloadQuota, user_id)
     if row is not None:
-        return QuotaLimits(row.max_bytes, row.max_concurrent, row.max_jobs, row.unlimited)
+        # Floor max_concurrent at 1 (admin_set_quota already does on write; the sysconfig default's
+        # min is 1). A stored 0 is nonsensical for a concurrency limit AND would diverge the two
+        # "at limit" checks — the worker's claim-window exclusion vs at_concurrency_limit — since a
+        # 0-limit user is "capped" with zero running jobs, which the SQL exclusion can't see. Keep
+        # them consistent by never letting the effective limit drop below 1. (To BLOCK a user's
+        # downloads, suspend them or set max_jobs=0, not max_concurrent=0.)
+        return QuotaLimits(row.max_bytes, max(1, row.max_concurrent), row.max_jobs, row.unlimited)
     return QuotaLimits(
         max_bytes=sysconfig.get_int(db, "download_default_max_bytes"),
-        max_concurrent=sysconfig.get_int(db, "download_default_max_concurrent"),
+        max_concurrent=max(1, sysconfig.get_int(db, "download_default_max_concurrent")),
         max_jobs=sysconfig.get_int(db, "download_default_max_jobs"),
         unlimited=False,
     )