[Bug] DB migration silently swallows errors in ALTER TABLE catch blocks

## Summary

`packages/desktop/src/main/db/index.ts` — several `ALTER TABLE ADD COLUMN` calls use empty `catch {}` blocks:

```typescript
for (const col of ['session_key TEXT', 'agent_id TEXT', 'run_id TEXT']) {
try {
sqlite.exec(\`ALTER TABLE messages ADD COLUMN \${col}\`);
} catch {}
}
```

While "duplicate column" errors are expected and safe to ignore, other errors (disk full, permission denied, schema corruption) are also silently swallowed with no logging.

## Expected behavior

Only suppress the expected "duplicate column" error, log unexpected ones:

```typescript
} catch (e: unknown) {
const msg = e instanceof Error ? e.message : String(e);
if (!msg.includes('duplicate column')) {
console.error(\`[db] migration failed for \${col}:\`, msg);
}
}
```

## Files

- `packages/desktop/src/main/db/index.ts` — migration section (line ~89)

## Context

Introduced in PR #210.

Start in packages/desktop/src/main/db/index.ts at the migration section around line 89 and inspect the ALTER TABLE ADD COLUMN catch blocks. Verify that expected duplicate-column errors remain suppressed while other migration errors are logged with the column context; the change is done when no unexpected ALTER TABLE failure is silently ignored.

Written by the indexing model from the issue text.