fix(indexer): remove ensureSchema() and add startup schema-version guard

[Chucks1093]

Closes #1176

What changed

1. ensureSchema() removed from src/index.ts

The function ran 15+ CREATE TABLE IF NOT EXISTS / CREATE INDEX IF NOT EXISTS statements on every process startup, bypassing the versioned migration suite in services/indexer/migrations/. Problems:

• Silently created old schema on fresh deployments that hadn't yet had migrations applied
• CREATE TABLE IF NOT EXISTS is a no-op on existing tables, so additive column changes from migrations were never applied to existing deployments
• Acquired AccessShareLock on pg_class on every startup
• No rollback path

2. src/schema-version.ts — startup guard

New assertSchemaVersion(pool) function replaces ensureSchema() in main(). It queries pg_tables and information_schema.columns to verify that all sentinel tables and columns are present, and calls process.exit(1) with a descriptive error message if any are missing:

DB schema is out of date — run migrations before starting the indexer:
  bash services/indexer/migrate.sh
  (or: docker compose run --rm migrate)

Sentinel tables checked: raw_events, indexer_cursor, indexer_state, device_tokens, sent_notifications, blocks, dm_keys, notification_preferences (011 — forward-progress sentinel).
Sentinel column checked: posts.content_tsv (added by 009_posts_fts).

3. docker-compose.yml — migrate init service

Added a migrate one-shot service that applies all .sql files in filename order before the indexer starts:

migrate:
  image: postgres:16-alpine
  restart: "no"
  depends_on:
    postgres:
      condition: service_healthy
  ...

indexer:
  depends_on:
    postgres:
      condition: service_healthy
    migrate:
      condition: service_completed_successfully

The postgres service's initdb.d mount is removed — migrations are now always applied explicitly by the migrate service, not implicitly on first boot.

4. migrate.sh — bare-metal migration runner

New services/indexer/migrate.sh for CI / bare-metal deployments (no Docker required):

DATABASE_URL=postgresql://user:pass@host/db bash services/indexer/migrate.sh

5. tests/migrations/test-migrations.sh — Step 10

New step verifies the assertSchemaVersion() sentinel invariants after the full migration run, and confirms the sentinel table (notification_preferences) is absent on a partially-migrated DB (negative test).

6. migrations/README.md updated

Removed the ensureSchema() paragraph and replaced it with instructions for running migrations via docker compose up or migrate.sh.

Tests

src/__tests__/schema-version.test.ts — 5 Jest cases:

• Resolves cleanly when all sentinel tables and columns are present
• Calls process.exit(1) when a required table is missing
• Calls process.exit(1) when posts.content_tsv is missing
• Reports all missing tables in a single error call
• Passes when the table list is a superset of the required tables

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[devJaja]

Nice work @Chucks1093

Kindly fix all the ci checks failure

[devJaja]

@Chucks1093

Resolve conflicts