Replace the try-and-ignore-duplicate-column approach in apply_migrations
with proper schema versioning using a shared schema_versions table.
## mechanism
New function: db::apply_versioned_migrations(conn, subsystem,
legacy_column, migrations). Tracks the applied-migration count in a
schema_versions table (one row per subsystem key). Only migrations past
the stored version run.
Legacy detection: pre-versioning databases have no schema_versions row.
The legacy_column tuple (table, column) identifies a column that exists
only in a fully-migrated legacy database. If present, all known
migrations are skipped. If absent, migrations start from 0.
## stores migrated
- broker: removes bespoke ensure_message_columns / ensure_reminder_columns.
Unified into BROKER_MIGRATIONS (v1-v5). Legacy detector: messages.priority
(added in the last pre-versioning migration).
- approvals: 4 historical migrations (v1-v4). Legacy detector:
approvals.submitter.
- operator_questions: 3 historical migrations (v1-v3). Legacy detector:
operator_questions.target.
- scheduled_prompts: 1 historical migration (v1). Legacy detector:
scheduled_prompts.paused_at_unix.
apply_migrations removed (no callers).
## tests (db.rs)
- fresh_install_runs_all_migrations
- legacy_install_skips_all_migrations
- partial_migration_resumes_from_version
- already_at_latest_is_noop
- multiple_stores_in_same_db
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