v0.11.0

Lifecycle webhooks (`[webhooks]`) and the CDR webhook (`[cdr.webhook]`) share one delivery transport, which now gains authenticity, idempotency, durability, and delivery metrics. **All additive** — the JSON bodies are unchanged, so the webhook and CDR schema `version`s are **not** bumped.

## Added
- **Idempotency.** Every delivery carries `X-SiphonAI-Event-Id` (+ an `Idempotency-Key` alias) — a UUIDv4 **stable across retries and any spool replay**. Delivery is at-least-once; receivers dedupe on this id.
- **Authenticity (opt-in `secret`).** With `[webhooks].secret` / `[cdr.webhook].secret` set, deliveries carry `X-SiphonAI-Signature: t=<unix>,v1=<hex>` — HMAC-SHA256 over `"<unix>.<raw-body>"`. The timestamp is inside the signed string, giving the receiver replay protection from a freshness window. `${VAR}`-expanded, never logged.
- **Durability (opt-in `spool_dir`).** With `[webhooks].spool_dir` / `[cdr.webhook].spool_dir` set, a delivery that exhausts the in-memory retry budget is persisted to disk and re-attempted by a background worker that **resumes after a daemon restart** (spool-on-failure — the happy path stays zero-disk-I/O). Oldest-first with capped backoff; `4xx`/poison entries are eventually discarded; a per-sink file cap bounds disk (drops the newest, never evicts an already-persisted entry). The directory is created + write-probed at startup, so a bad path fails the daemon loudly. Unset ⇒ today's best-effort behavior, unchanged.
- **Delivery metrics.** `siphon_ai_webhook_deliveries_total{sink,result}`, `siphon_ai_webhook_delivery_attempts_total{sink,outcome}`, `siphon_ai_webhook_spool_depth{sink}`, `siphon_ai_webhook_delivery_seconds{sink}` (`sink` ∈ `lifecycle` | `cdr`).

Both `secret` and `spool_dir` are **off by default**.

Docs: `docs/DEPLOY.md` → *Webhook delivery: signing, idempotency, durability* (with a receiver verification snippet); `docs/CONFIG.md` `[webhooks]` / `[cdr.webhook]`.

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