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/alembic/versions/0062_drop_redundant_idx.py

@@ -42,7 +42,7 @@ _REDUNDANT: list[tuple[str, str, str]] = [
 
 
 def upgrade() -> None:
-    for idx, table, _col in _REDUNDANT:
+    for idx, _table, _col in _REDUNDANT:
         # IF EXISTS: a DB whose schema was stamped from a create_all (tests) won't have these once
         # the models drop index=True, so a plain drop_index would error there.
         op.execute(f"DROP INDEX IF EXISTS {idx}")