fix(db): correct channel contact merged_id foreign key

[nguyenha935]

Summary

• Fix channel_contacts.merged_id FK to reference tenant_users(id).
• merged_id is written by MergeContacts(... tenantUserID) and resolved via channel_contacts.merged_id = tenant_users.id.
• The previous self-reference FK from migration 000083 is incompatible with the application data model and valid historical data.
• Add migration 000086 for already-migrated beta databases and update FK audit checks.

Surface parity

• Gateway/server: affected via PostgreSQL migration.
• Web UI/API contract: N/A, no request/response shape changes.
• CLI/runtime: N/A, no command or protocol changes.
• SQLite: N/A, SQLite schema does not enforce this FK; sqlite build still verified.

Tests

• Docker builder stage build succeeded with ENABLE_EMBEDUI=false.
• /usr/local/go/bin/go test ./internal/store/... ./internal/upgrade/...
• /usr/local/go/bin/go build -tags sqliteonly ./...
• /usr/local/go/bin/go vet ./...
• Restored a pre-fix DB dump into a temporary database and verified migration 000086 changes merged_id FK from channel_contacts(id) to tenant_users(id).

[clark-cant]

🦸‍♂️ Approved — Clean FK fix.

Summary: Corrects channel_contacts.merged_id FK to reference tenant_users(id) instead of the erroneous self-reference from migration 000083. Matches actual application data model (MergeContacts writes tenantUserID into merged_id).

Review:

• Migration up/down: ✅ correct — drops old constraint, adds proper FK with ON DELETE SET NULL
• Schema version bump: ✅ 85 → 86
• Audit SQL: ✅ updated to check against tenant_users + added tenant drift check
• CI: ✅ go, web, release-versioning all green
• Scope: ✅ small, focused (34+/5−, 4 files)
• Testing: ✅ author verified with pre-fix DB dump

Ship it.