Make deep backfill's Phase C (orphan-contact discovery) opt-in

[DiscoBard]

Summary

DeepBackfill runs three phases:

• Phase A — paginate INBOX/ARCHIVE/SPAM_BLOCKED, store conversations
• Phase B — for each discovered conversation, paginate messages
• Phase C — discoverFromContacts calls GetOrCreateConversation for every contact's phone number to surface threads not visible from folder listings

Phases A and B are pure history reads. Phase C has a write-shaped side effect: Google Messages treats GetOrCreateConversation as a thread-creation, so for each contact without a prior thread, an empty SMS thread is created on the user's phone. A user with several dozen contacts who runs deep backfill ends up with several dozen blank conversations they have to delete by hand.

This PR keeps Phase C available but makes it opt-in via a new env var:

OPENMESSAGES_BACKFILL_DISCOVER_ORPHANS=1

Default behavior changes from "Phase C runs on every deep backfill" to "Phase C runs only when explicitly enabled." Users who want the orphan-discovery behavior get it back with one env var; users running deep backfill purely as a history sync no longer get unintended new threads.

Implementation

• internal/app/backfill.go — add orphanContactDiscoveryEnabled() helper (parses 1/true/yes/on, case- and whitespace-tolerant). Gate the Phase C call in deepBackfill() on this. Off-path logs an informational line referencing the env var.
• internal/app/backfill_test.go:
  • TestOrphanContactDiscoveryEnabled — table-driven coverage of the parser (truthy, falsy, mixed case, whitespace, unrecognized values)
  • TestDeepBackfillSkipsPhaseCWhenOptOut — asserts no contact-discovery side effects on the default-off path (no contacts checked, no orphan conversations stored)
  • TestDeepBackfillContactDiscovery, TestDeepBackfillContactDiscoverySkipsAlreadySeen, TestDeepBackfillGetOrCreateError — opt in via t.Setenv since they specifically exercise Phase C
• README.md — document the new env var with a clear warning about the empty-thread side effect

Test plan

• go build ./... — clean
• go test ./internal/app/ ./internal/client/ — passes (existing + new helper test + new opt-out regression test)
• Manual: run with the env unset, confirm Phase C log line appears and no new threads on phone after deep backfill
• Manual: run with OPENMESSAGES_BACKFILL_DISCOVER_ORPHANS=1, confirm previous behavior is preserved

Compat

No API change. Default behavior changes; users relying on Phase C running automatically will need to set the env var. The off-path log line tells them exactly what to do.

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