feat(memory-engine): add off-peak consolidation windows

[xiaoxiaoHe-E]

Summary

Implements #2762 — restrict the consolidation background job (Hindsight's most LLM-heavy operation) to a configurable daily off-peak window (e.g. 01:00–06:00), instead of firing whenever facts become eligible.

Tasks that land outside the window are deferred, not failed: the worker holds them as pending with next_retry_at set to the next window open, and the backlog drains automatically when the window reopens (claim SQL next_retry_at <= NOW()).

Changes

• New static config trio (config.py):
  • HINDSIGHT_API_CONSOLIDATION_WINDOW_START / HINDSIGHT_API_CONSOLIDATION_WINDOW_END — HH:MM (24h) daily window boundaries; must be set together. START > END = cross-midnight window (e.g. 22:00–06:00); START == END = always-open. Unset = always-on (previous behaviour).
  • HINDSIGHT_API_CONSOLIDATION_WINDOW_TZ — IANA timezone the boundaries are evaluated in; unknown names fall back to UTC with a warning.
• engine/consolidation_window.py (new) — pure, deterministic next_window_open() schedule arithmetic (normal / cross-midnight / DST-aware), unit-tested.
• _handle_consolidation gate — defers via the existing DeferOperation mechanism (no retry_count bump, no error_message) while the window is closed.
• Wires the previously-dead validate_consolidate extension hook into the same deferral path — it was defined on the validator interface but never called for consolidation. A rejection is now a deferral (retry-backoff), not an HTTP 403.
• Sync task backend safeguard — on SyncTaskBackend (tests / CLI / embedded single-process) there is no worker queue to re-claim a deferred task, so raising DeferOperation would 500 the HTTP caller and strand a pending op that blocks banks_needing_consolidation() from re-submitting the bank. There it logs and runs now (window closed) / skips (hook rejection) instead.

Notes

• The manual sync path run_consolidation() (explicit user trigger) is intentionally not gated by the window — the window applies to the async background job.
• Window boundaries use wall-clock replace(); on DST spring-forward/fall-back transition days the boundary can shift by one hour (window times are typically in the early morning).

Testing

• 21 unit tests: schedule arithmetic (next_window_open) + env parsing (_parse_consolidation_window*).
• 7 integration tests: worker deferral semantics (no retry bump / no error, next_retry_at = next window open), reconcile non-resubmission, extension rejection deferral, sync-backend run-now/skip behaviour.
• ruff, ty, and the repo pre-commit hooks (lint.sh, generate-docs-skill.sh, check-unused.sh) all pass.

Closes #2762