UC5: notify removed PP backend on provider removal

[AquiGorka]

What

Part of the UC5 PP removal & continuity slice (council + provider stacks honour a PP removal end-to-end). This is the council-platform piece.

When the council channel-auth contract emits ProviderRemoved, the event-watcher already marks the provider REMOVED. This adds the outbound notify to the removed PP's backend so it reacts immediately rather than waiting for its own next poll.

How

• New notify-provider-removed.ts: fire-and-forget POST {providerUrl}/api/v1/council/removed with { councilId, publicKey, ledger }.
• Called from the provider_removed branch of applyEvent, guarded by the ACTIVE→REMOVED transition + presence of provider_url.
• The notice is a low-trust live signal — the body is a hint, not an authority. The PP converges by re-querying the council's authoritative /public/provider/membership-status endpoint, so no shared secret is required and a spurious/replayed POST can only make a PP re-confirm its own status.
• Never awaited → cannot stall or roll back the watcher's atomic poll commit (same shape as the existing notifyDiscord).
• Idempotency / boot replay: only the ACTIVE→REMOVED transition notifies, so event re-delivery and a removal observed during downtime (replayed from the ledger cursor on boot) ping the PP exactly once.
• Extracted applyEvent into env-free apply-event.ts so the transition logic is unit-testable without loading DATABASE_URL.

Tests

src/core/service/event-watcher/notify-on-removal.test.ts: active removal notifies once; already-REMOVED re-delivery/boot-replay does not re-notify; no provider_url → dormant; unknown provider → no-op.

Full PR pipeline run locally on the pinned deno 2.8.2: fmt --check, lint, check src/main.ts, test:unit (58), test:integration (185) — all green.

Version: 0.6.2 → 0.6.3.

Coordination

Lands together with the provider-platform inbound notify endpoint and the provider-stack (Rust standin) removal-honouring PRs. The contract needs no change — remove_provider already emits ProviderRemoved.