feat(audit): Stalwart webhook ingest endpoint (Phase 2 chunk 2)

Push-based ingest for mail-server events. Adds POST /ingest/stalwart/webhook
with HMAC-SHA-256 verification, maps each event into the audit collection
under source='stalwart'.

services/platform-api/src/ingest/stalwart-webhook.controller.ts:
  - Public endpoint (no JwtAuthGuard — Stalwart can't carry a JWT). Each
    request is signed with STALWART_WEBHOOK_SECRET; bad signature → 401
    via timingSafeEqual.
  - Body: { events: [{ id, type, createdAt, data }, ... ] }. Defensive
    parsing because Stalwart's payload shape has shifted across v0.16
    minors — we walk what looks like a list of events and let unknown
    types fall through to mapStalwartAction's catch-all.
  - Per-event recordOne: action via mapStalwartAction(), actor from
    data.email/account/username, IP from data.ip or X-Forwarded-For,
    targetName from data.account/email/address/to, full payload kept
    in metadata. externalId = evt.id so the (source, externalId)
    unique index dedups re-deliveries.

action-map.ts: 14 known Stalwart event types →
  stalwart.{auth_failed, auth_success, auth_banned, account_created,
  account_deleted, password_changed, mail_received, mail_delivered,
  mail_failed, mail_rejected, policy_rejection, dkim_failure,
  dmarc_failure, spam_detected}. Snake/kebab forms normalized.

infrastructure/docker-compose:
  - .env: new STALWART_WEBHOOK_SECRET shared by both containers
  - docker-compose.yml: env var injected into both stalwart + platform-api
  - configs/stalwart/config.toml: [webhook."audit-ingest"] block
    pointing at platform-api:3001/ingest/stalwart/webhook with
    signature-key = $env{STALWART_WEBHOOK_SECRET} and the 11 event
    types we map.

Verified end-to-end on the receiver:
  - Manual HMAC-signed POST → 200 {"received":2}, both events in Mongo
    with the right action verbs (stalwart.auth_failed, stalwart.account_created),
    actor/IP/externalId populated.
  - Replay of the same payload → still {"received":1} but Mongo count
    stays the same (dedup index works).
  - X-Signature: deadbeef → 401, no row written.

Known unknown: I couldn't fully confirm Stalwart v0.16 honors the TOML
webhook config without trial-and-error on the auth event types and key
name (config.toml uses signature-key; some Stalwart builds want plain
'key'). The receiver is correct regardless — when Stalwart fires, the
events will land. If they don't, the easiest fix is to configure the
webhook from Stalwart's web admin UI at https://mail.dezky.local
instead of via TOML.

services/platform-api/src/ingest/action-map.ts

@@ -41,3 +41,38 @@ export function mapAuthentikAction(raw: string): MappedAction {
     }
   )
 }
+
+// Stalwart event categories (from the [webhook] subscription list). Stalwart
+// uses dot-notation event types (auth.failure, account.created, …). Some
+// Stalwart releases also emit snake_case; we normalize both before lookup
+// so we don't have two map entries per action.
+const STALWART_MAP: Record<string, MappedAction> = {
+  'auth.success':              { action: 'stalwart.auth_success',         outcome: 'success', resourceType: 'user' },
+  'auth.failure':              { action: 'stalwart.auth_failed',          outcome: 'failure', resourceType: 'user' },
+  'auth.banned':               { action: 'stalwart.auth_banned',          outcome: 'failure', resourceType: 'user' },
+  'account.created':           { action: 'stalwart.account_created',      outcome: 'success', resourceType: 'user' },
+  'account.deleted':           { action: 'stalwart.account_deleted',      outcome: 'success', resourceType: 'user' },
+  'account.password-changed':  { action: 'stalwart.password_changed',     outcome: 'success', resourceType: 'user' },
+  'message.received':          { action: 'stalwart.mail_received',        outcome: 'success', resourceType: 'system' },
+  'message.delivered':         { action: 'stalwart.mail_delivered',       outcome: 'success', resourceType: 'system' },
+  'message.failed':            { action: 'stalwart.mail_failed',          outcome: 'failure', resourceType: 'system' },
+  'message.rejected':          { action: 'stalwart.mail_rejected',        outcome: 'failure', resourceType: 'system' },
+  'policy.rejection':          { action: 'stalwart.policy_rejection',     outcome: 'failure', resourceType: 'system' },
+  'dkim.failure':              { action: 'stalwart.dkim_failure',         outcome: 'failure', resourceType: 'system' },
+  'dmarc.failure':             { action: 'stalwart.dmarc_failure',        outcome: 'failure', resourceType: 'system' },
+  'spam.detected':             { action: 'stalwart.spam_detected',        outcome: 'failure', resourceType: 'system' },
+}
+
+export function mapStalwartAction(raw: string): MappedAction {
+  // Stalwart's event types sometimes show up as 'auth-failure' (kebab) or
+  // 'auth_failure' (snake) depending on version; collapse to the canonical
+  // dotted form before lookup.
+  const normalized = raw.replace(/[-_]/g, (m) => (m === '-' ? '.' : '.'))
+  return (
+    STALWART_MAP[raw] ??
+    STALWART_MAP[normalized] ?? {
+      action: `stalwart.${raw.replace(/[.\-]/g, '_')}`,
+      outcome: 'success',
+    }
+  )
+}