Clean up the query data nothing refers to, and survive jobs without a result (#68)

[r0ny123]

Closes #68 (considerations for DbCleanup: orphan query samples and functions without a job, a compact afterwards).

What the cleanup did, and the bug in it

doDbCleanup deleted query samples and query jobs older than STORAGE_MONGODB_CLEANUP_TTL, which already covers query samples whose job is gone (they age out by their own timestamp). It read result["info"]["sample"] off every finished and failed query job, though, and a failed job that died before matching has no result, so the first such job took the whole cleanup down with a TypeError. On an instance with STORAGE_MONGODB_ENABLE_CLEANUP that means the cleanup never completes again once one query job has failed early.

Changes

• A job without a result is nothing to protect and nothing to collect: it is deleted once older than the TTL and kept otherwise. Finished jobs are dated by finished_at, failed ones by started_at, a job that never started counts as old.
• After the TTL pass, deleteOrphanedQueryData() on the storage removes the query functions whose query sample is gone and the query disassembly (query_xcfg) whose function is gone: what an interrupted deletion or a job deleted without its sample leaves behind. The query collections are bounded by the TTL, so listing their ids is affordable; the memory backend drops orphaned query functions the same way.
• STORAGE_MONGODB_COMPACT_AFTER_CLEANUP (default off) runs MongoDB's compact on query_samples, query_functions, query_xcfg, fs.files and fs.chunks afterwards, so the freed space goes back to the file system. It needs the compact privilege; a refusal is reported per collection, not raised, since the cleanup itself has succeeded by then.
• The job now answers a report: samples and jobs deleted, orphans removed, and the compact outcome when it ran.

Verification

• tests/testWorkerCleanup.py (3 tests, worker with fakes): an old failed job without a result is deleted and a recent one kept, a job that never started counts as old; a recent job protects its sample while an old job takes its old sample along and a recent sample is kept; orphan removal always runs and compaction only when configured, with both reflected in the report.
• tests/testStorage.py (Mongo): orphaned query functions and disassembly are deleted while the query sample's own are kept, and a second run finds nothing; compact answers per collection against the real MongoDB.
• Full suite: 208 passed; ruff check, ruff format --check, ty check clean.
• Live verification on the MongoDB-backed instance follows in a comment.

[r0ny123]

Live verification on the MongoDB-backed instance (server and worker restarted on this change):

Planted the residue the issue is about: two query functions whose query sample does not exist, two query disassembly documents (one for an orphan function, one for no function at all), and a failed getMatchesForUnmappedBinary job from nine days ago with attempts_left: 0 and no result, which is the shape that used to take the cleanup down.

Scheduled doDbCleanup through the index. The job finished and answered:

{'num_query_samples_deleted': 0, 'num_query_jobs_deleted': 1, 'orphans': {'query_functions': 2, 'query_xcfg': 2}}

Afterwards query_functions and query_xcfg were empty, the failed job was gone, and the four corpus samples with their 1158 functions were untouched.