fix(db): normalize conversation membership

[KIDA-MNESIA]

Summary

• fix AR-H3 by making tenant-constrained conversation_members rows the sole authorization and routing source while retaining conversations.members only as a rebuildable API projection
• serialize invite, leave, kick, onboarding, and conversation creation with participant and conversation locks; commit membership, audit message, sequence, and realtime outbox changes in one PostgreSQL transaction
• migrate production membership queries away from JSONB containment, add a source guard plus two-client race coverage, and record the design in ADR 0004

Migration and compatibility

• add immutable migration 0002 with exact checksum validation, composite tenant foreign keys, deterministic backfill, and required promotion indexes
• fail closed on unknown or cross-tenant member ids while stripping legacy external:* email author markers that were never authorization principals
• keep an expand-phase trigger that translates writes from the previous application revision into normalized rows; a later contract migration can reject projection writes and remove the old JSONB index after the rollback window closes

Tests

• npm run lint (466 files)
• npm run typecheck
• npm run server:typecheck
• npm run build
• npm test (1066 passed, 4 platform-gated skips)
• npm run test:integration (284 passed, 5 credential-gated Resend skips)
• focused agenda, normalization, email, membership, and runtime suites (49 passed)
• fresh PostgreSQL v2 migration smoke (63 tables, exact v1/v2 checksums, composite FKs, required indexes)
• npm run guard:big-brain
• npm run guard:llm-tracked
• npm run guard:engine-registry