feat(slack-app phase 3a): /slack/events webhook + email-match identity

[TS00]

Summary

Phase 3a of the Slack app (full design: `docs/eng-plan-slack-app-v1.md`, parent memory `d959bc61`). Stands up the events webhook, signature verification, and the identity resolver. The bot now responds to @mentions in channels and DMs with a canned reply confirming who it sees you as. Phase 3b (next PR) swaps the canned reply for the Anthropic agent loop with memory tools.

What's new

• `POST /slack/events` route — public (signature-verified, not Bearer-auth). preParsing hook captures the raw body so HMAC sees what Slack signed. URL verification echoes the challenge synchronously. `event_callback` acks 200 immediately and runs the handler as a fire-and-forget Promise (Slack's 3s deadline).
• `src/slack-api.ts` — typed thin wrappers around `users.info`, `chat.postMessage`, `chat.postEphemeral`. `SlackResult` discriminated union for clean error handling.
• `src/slack-identity.ts` — `resolveSlackUserToReflectUser`. One rule: the Slack user's email must lookup-match a Reflect user bound to the workspace's team (or the workspace's solo Reflect user). All refusal paths return a clean human-readable reason — no fallback link, no manual mapping UI (per Van's call earlier in the session).
• `src/slack-events-handler.ts` — sync filter + async handler. Skips bot/edit/non-im events, posts ephemeral refusals in channels (don't shame the requester publicly) and regular replies in DMs. Audit-logs `slack.message.handled` or `slack.auth_refused` with both the Slack user ID and the resolved Reflect user ID.

Tests

347/347 backend tests green (was 335; +12). Coverage:

• HTTP integration: URL verification, signature gate (missing/wrong/stale), event_callback ack-200 contract.
• `processSlackEvent` unit: every ignored event-type path (bot_message, message_changed/deleted, public-channel non-mention, incomplete payloads, self-app messages).

End-to-end async handler tests with a mocked Slack are deferred to Phase 3b. Cross-process mocking (test process vs. server process) is fragile; manual smoke against the live `Reflect Dev` app is faster and more honest at this stage.

Manual step needed AFTER merge + dev deploy

The Slack app's Event Subscriptions are still OFF. After dev deploy lands:

1. Open the `Reflect Dev` app at https://api.slack.com/apps
2. Go to Event Subscriptions → toggle ON
3. Set Request URL to `https://api-dev.reflectmemory.com/slack/events\`
4. Slack will hit it for a URL verification challenge — should show ✓ Verified within seconds (this PR's code handles it)
5. Under Subscribe to bot events, add: `app_mention` and `message.im`
6. Save changes (Slack will prompt to reinstall the app to grant the new event scopes if needed)
7. Add the bot to a channel: `/invite @reflect-dev`
8. `@reflect-dev hi` — expect a canned reply confirming your email match

Test plan

• CI green
• Dev deploy succeeds, service boots cleanly
• Event Subscriptions verification handshake passes in the Slack admin UI
• @mention in a channel returns the Phase 3a reply ("Hi — I see you ()…")
• DM the bot — same reply, but as a regular thread reply (not ephemeral)
• @mention as a stranger (Slack user with email not in Reflect) → ephemeral refusal in channel; audit `slack.auth_refused` recorded

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