Tracer: bind a connected Lark app; DMs and mentions reach the Inbox; the reply goes back in place #1108

Parent: #1107 · ADR-0142

What to build

The narrowest end-to-end path: an already-connected Lark app, bound from the Bot's External identities entry, delivers DMs and group @mentions to that Bot's Inbox, and the Bot replies in the original conversation. There are no delivery targets, no conversation authorization and no reception switch.

• Host, Messaging:
  • The Binding's account-level registration (today the Lark pairing control registration in inbound.ts startControl) becomes the reception owner for default traffic.
  • Dispatch order: control intake → active entry → default traffic → acknowledge without admission. The durable-block step before default traffic arrives with blocks in Conversation list: mute, rules, block that sticks, allow again #1109.
  • The first admitted DM or mention commits an implicit entry in messaging_grants (origin: 'implicit', conversation kind and ID from the authenticated event, no target ref or digest) in the same transaction as the Source Event and Inbox Admission.
  • Validity of implicit entries checks Binding, fingerprint and the conversation itself, instead of re-inspecting a saved target.
  • bridge_reply works on sources admitted this way.
• Schema (generation 61):
  • Add messaging_bindings.new_conversations (auto | ask). Existing and new rows get auto (Vain, 2026-10-07).
  • Add a partial unique index allowing one active entry per Binding, conversation kind and conversation ID.
  • Existing Grants get origin: 'explicit' and keep their behavior.
• Client:
  • External identities: Bind identity becomes Bind app. The platform is fixed to Lark in this slice, and the dialog lists connected Lark apps.
  • After the commit, the dialog shows the registration's real readiness (connecting / ready / unavailable).
  • Each app row shows a conversation count and opens a read-only list of its conversations: name or ID, DM or group, last message time.

Acceptance criteria

• With a connected Lark app and no saved targets or Grants: Bind app → DM the app → one Source Event and Inbox Admission for that Bot → bridge_reply lands in the same DM.
• @mention the app in a group it belongs to → Inbox → the reply lands in the same group or topic. An unmentioned group message is not admitted.
• /pair in a DM is still handled by pairing and is not admitted.
• Existing explicit Grants keep exactly their current behavior: collection, routes, policies, revisions.
• An existing Binding migrates to auto and admits new DMs and mentions. A Binding switched to ask admits nothing from new conversations.
• Redelivery creates no duplicate entry or admission. Restart preserves entries and keeps admitting.
• Identity pause and unbind stop admission immediately. An unbound app's entries are invalidated.
• Migration test covers generation 60 → 61 with existing bindings, grants and revoked grants, plus the snapshot and refusal of older schema writers.
• Real Lark demonstration with the packaged Provider, DM and group, with sanitized screenshots. zh/en, light/dark before/after evidence of the Bind app dialog and the conversation list.

Blocked by

None.