fix: prevent stale capacity poisoning and deepen recent-only refill scans

Keep ordinary capacity races from lowering persistent per-account limits.
Deepen recent-only discovery within the existing age and quality gates.
Add offline regressions for recovery and reserved-slot refill.

README.md

@@ -301,6 +301,16 @@ model/GPU combinations, the same run progressively expands discovery to the last
 7 days, the last 30 days, and finally older history (up to 3,000 models). Scanning
 stops as soon as enough replacement candidates have been found.
 
+When only recent-model reserved positions remain, discovery also deepens the
+same recent window up to the configured scan cap (3,000 by default). It never
+admits older models into those positions or lowers the quality gates. This avoids
+rechecking only an exhausted short prefix on every poll.
+
+An ordinary account-capacity rejection temporarily marks that account full and
+tries the next account. It does not lower the persisted capacity using a cached
+active count: another worker may have filled the queue since that count was
+read. Downward capacity learning remains confined to weekly calibration.
+
 Model verification results are reused for 15 minutes across poll cycles, and a
 locally failed model/GPU pair cools down for 24 hours instead of being excluded
 forever. The `[scan]` lines show every expansion stage, while `[daily] wave_done`