fix(notifications): make pipeline.flush() drain in-flight dispatches

[lesquel]

Summary

Fixes the 3rd and final latent bug from the #25 comment (surfaced by #28's characterization tests).

pipeline.flush() was an empty no-op. The genuinely fire-and-forget dispatches (telegram/push/channels fan-out) were never tracked, so a notification in flight when the process shut down was silently dropped — exactly the AGENTS.md §"No silent failures" anti-pattern.

Fix

• Track each already-.catch()-chained fire-and-forget promise in an inFlight Set via a void track() helper (.finally removes it). 5 sites: telegram + push + channels in notifyPermissionPending; channels fan-out in notifySessionIdle / notifySessionError. The two await-ed telegram calls in idle/error are not fire-and-forget → untouched.
• flush() now awaits the in-flight set with a bounded timeout (FLUSH_TIMEOUT_MS = 5_000) via Promise.race. On timeout it audits notifications.flush_timeout with the pending count (observable, never silent), always clearTimeouts (no leaked handle), fast-paths when nothing is in flight, and never throws/hangs beyond the bound.

Guarantees (fresh-review verified)

• Hot path unchanged: track() is void; no new await added to any notify function; notifyPermissionPending returns the same boolean at the same point → the permission hook's blocking decision timing is identical to main.
• No unhandled-rejection vector: only post-.catch() (never-rejecting) promises are tracked.
• Shutdown wiring intact: server/index.ts still await notifications.flush() in the same position; now the bound is meaningful.
• Test seam: optional createNotificationService(deps, { flushTimeoutMs }) — production caller unchanged, defaults to 5s.

Testing

• The test(integrations,notifications): cover native hooks + pipeline #28 no-op characterization test was replaced with 4 tests that genuinely fail on main's no-op flush (fresh review verified them discriminating, not tautological); timeout test uses an injected 50ms bound (no real 5s wait, deterministic).
• bun test 627/0 · bunx tsc --noEmit clean · scope = pipeline.ts + pipeline.test.ts only
• Independent fresh-context adversarial review: APPROVE, no blocking issues (all 7 checks pass; one cosmetic nit — stale header comment — fixed).

Independent off main. No overlap with #33 wave-2 PRs or #40.

Refs #25 — completes all 3 latent items (with #40). #25's remaining lint/arch items were shipped earlier (#27/#30-32).