connector: put calls and voicemails in the SMS chat

[iFixRobots]

Fixes #3.

Google Voice gives every call and voicemail its own thread (c.<id>), separate from the SMS thread with the same person (t.<number>). The bridge maps threads to portals one-to-one, so every call becomes a new chat.

What changes

• Routing. A call or voicemail thread with exactly one phone number now goes into that number's SMS portal, t.<number>. If there's no SMS chat yet, it creates one, so all calls from someone you've never texted still end up in a single chat. Group threads and threads without a normal phone number are unchanged.
• Thread tracking. Each merged message records its source thread in message metadata (thread_id).
  • Read receipts mark the merged call and voicemail threads read in Google Voice, not just the SMS thread.
  • Deleting the chat also deletes the merged threads, so they don't bring the chat back on the next fetch. A failure on a merged thread is only logged, so one bad thread can't make the chat undeletable.
• Forward-only. Existing per-call portals are left as they are. A call thread whose latest message is already bridged is skipped. Backfills of merged threads use AggressiveDeduplication, because message IDs are unique per login across portals.