* fix: batch-limit stale managed object cleanup to prevent 300K row UPDATE (#25257)
* Add STALE_OBJECT_CLEANUP_BATCH_SIZE constant
Configurable batch limit (default 1000) for stale managed object cleanup,
preventing unbounded UPDATE queries from hitting 300K+ rows at once.
* Batch-limit stale managed object cleanup with single bounded SQL query
Two fixes to _cleanup_stale_managed_objects:
1. Replace unbounded update_many with a single execute_raw using a
subquery LIMIT, capping each poll cycle to STALE_OBJECT_CLEANUP_BATCH_SIZE
rows. Zero rows loaded into Python memory — everything stays in Postgres.
Uses the same PostgreSQL raw-SQL pattern as spend_log_cleanup.py
(the proxy requires PostgreSQL per schema.prisma).
2. Extract _expire_stale_rows as a separate method for testability.
Keeps the file_purpose='response' filter to avoid incorrectly expiring
long-running batch or fine-tune jobs that legitimately exceed the
staleness cutoff.
* docs: add STALE_OBJECT_CLEANUP_BATCH_SIZE to env vars reference
* test: remove deprecated embed-english-v2.0 cohere embedding tests
