feat(store): Postgres backend — pgx driver, Ent schema parity, full store port, LISTEN/NOTIFY

[ptone]

Summary

Adds Postgres as a supported database backend alongside SQLite. SQLite remains the default — no existing deployments are affected unless they opt in via database.driver: postgres.

This is the foundation PR. PRs for multi-node broker dispatch and NFS workspace depend on this merging first.

What's included

• P0 — Switch from lib/pq → pgx/v5 stdlib; connection pool config (MaxOpenConns, ConnMaxIdleTime, ConnMaxLifetime); CRUD-parity test harness; Cloud Monitoring scaffolding for LISTEN/NOTIFY metrics
• P1 — 23 new Ent schemas covering all ~25 raw-SQL tables (full table parity with pkg/store/sqlite/)
• P2 — Port all store domains to Ent entadapter: user, allowlist/invite, schedule, maintenance, message, notification, GCP/GitHub/token, secret/env_var, template/harness_config, project/broker, agent. Remove raw-SQL store. Collapse dual-DB into single Ent store.
• P3 — Postgres LISTEN/NOTIFY event publisher (PostgresEventPublisher); dialect-aware multi-replica concurrency primitives (advisory locks, singleton scheduler); parameterized store tests over {sqlite, postgres}
• Pool hardening — ConnMaxIdleTime (5 min default), TCP keepalive GUCs, advisory-lock tick-skip on connection error, bounded lock-checkout timeout
• Migration tooling — In-process migration α (legacy raw-SQL hub.db → Ent-on-SQLite); migration β tool (Ent-SQLite → Ent-Postgres)

Test plan

• go build ./... passes (verified on branch)
• go test ./pkg/ent/... ./pkg/store/... passes
• Integration test: go test -tags integration ./pkg/store/integrationtest/... against a Postgres instance
• Deploy with database.driver: sqlite (default) — confirm zero behavior change
• Deploy with database.driver: postgres + DSN — confirm hub boots and operates normally

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[gemini-code-assist]

Code Review

This pull request implements database-related enhancements, including PostgreSQL support, database connection pooling, SQLite-to-PostgreSQL migration commands, and a test fixture generator. Feedback on these changes highlights potential goroutine and connection leaks due to the use of context.Background() when initializing the event publisher and running database migrations. Additionally, the DSN parsing logic for SQLite should be refined to handle file:// prefixes correctly to avoid leaving triple slashes in file paths.

[gemini-code-assist]

Using context.Background() here when initializing the event publisher can lead to goroutine and connection leaks. The Postgres event publisher starts a background listener for LISTEN/NOTIFY that runs indefinitely until the context is cancelled. Since context.Background() is never cancelled, this connection and its associated goroutines will leak when the server shuts down or during testing. Consider refactoring initWebServer to accept a context.Context parameter and pass it here.

[gemini-code-assist]

Using context.Background() for database migrations prevents graceful cancellation (e.g., if the operator interrupts the startup process with Ctrl+C). Consider refactoring initStore to accept a context.Context parameter so that s.Migrate and s.Ping can be cancelled properly.

[gemini-code-assist]

When parsing a file:/// DSN, trimming only the file: prefix leaves triple slashes (e.g., ///var/lib/...), which can cause issues when passed to file system operations like os.Remove. Handling file:// explicitly avoids this.

	case strings.HasPrefix(raw, "file://"):
		path = strings.TrimPrefix(raw, "file://")
	case strings.HasPrefix(raw, "file:"):
		path = strings.TrimPrefix(raw, "file:")