ops: disable epoch_head backfill in the production compose

[samgbafa]

TC-1274. Sets TINYCLOUD_EPOCH_HEAD_BACKFILL__ENABLED: "false" in docker-compose.dstack-postgres.yaml so the #343 epoch_head backfill does not start at boot. Writes stay correct without it; re-enable once DB CPU has headroom (pace keys: TINYCLOUD_EPOCH_HEAD_BACKFILL__SPACES_PER_SECOND, __PROGRESS_EVERY).

• Compose lock re-pinned: 6563d51b…f04e -> c33c23021589163b62a71ef245b7194717711922027629f155bdd5cff806b8c7 (docker.yml N0 hash, docs/releasing.md, tc-959 runbook, scripts/ci/fixtures/tc959-n0-compose.yaml).
• Test epoch_head_backfill_enabled_reads_only_the_double_underscore_key builds build_config_figment(): default on; double underscore false => disabled; single underscore does not disable; SPACES_PER_SECOND is read.
• fix(TC-1274): bound SQL waits and add an /invoke deadline #347 also edits this compose: whichever lands second must re-pin.

[samgbafa]

APPROVE: see TC-1274 (reviewed exact head 1286793)