Ito QA identified the following failure during automated PR testing. Please investigate and propose a fix.

**Medium severity — Startup gate opens before replication viability is known**

**What failed:** The gate is released before replication startup is validated, so the process accepts external traffic during a transient window where overall startup is not yet viable.

- **Impact:** When replication startup fails, the server can briefly accept real client queries before shutting down, causing interrupted sessions and unreliable startup behavior for database users.
- **Steps to reproduce:**
  1. Start the server with an empty data directory so first-run default database initialization runs under the startup gate.
  2. Configure replication to a non-routable upstream (for example `10.255.255.1`) so `startReplication()` cannot establish viability promptly.
  3. Race external `psql` connections immediately after startup and observe successful queries before replication startup later fails/stalls.
- **Stub / mock content:** No stubs, mocks, or bypasses were applied for this test in the recorded run.
- **Code analysis:** In `server/server.go`, the PR-modified startup path calls `createDefaultDatabase(ssCfg, gate)`, then `gate.Release()`, and only afterward runs `startReplication(cfg, ssCfg)`. If replication initialization blocks or fails, external connections have already been allowed. Runtime evidence matches this ordering: logs show `Starting replication` followed by external `NewConnection` IDs 2-8, and captured `psql` queries (`SELECT 1`, `SELECT version()`) succeed during that window.
- **Why this is likely a bug:** The expected behavior for this test is that external acceptance is deferred until replication viability is known, or that temporary accepts are safely isolated. The current code does neither: it intentionally opens the gate first and then performs replication startup, which allows real client queries in an inconsistent startup state.

**Relevant code:**

`server/server.go:194-207`

~~~go
if initializeDefaultDatabase {
	err = createDefaultDatabase(ssCfg, gate)
	if err != nil {
		controller.Stop()
		return nil, err
	}
	// Release the startup gate to accept external connections now that init is finished
	gate.Release()
}

// TODO: shutdown replication cleanly when we stop the server
_, err = startReplication(cfg, ssCfg)
if err != nil {
	controller.Stop()
	return nil, err
}
~~~