BUG: IMAP connection pool exhaustion under load

[andrinoff]

Describe the bug: No connection pooling or limit on concurrent IMAP connections. Multiple operations can exhaust server connection limits. Server may refuse connections or ban client.

To reproduce:

1. Configure multiple accounts (5+)
2. Trigger simultaneous sync on all
3. Each spawns multiple IMAP connections
4. Server hits connection limit
5. Errors: "too many connections" or connection refused

Expected behavior:

• Implement connection pool with max limit
• Queue operations when pool full
• Reuse idle connections
• Respect server connection limits
• Graceful degradation on limit hit

Additional context:

• File: fetcher/fetcher.go
• Need connection pool implementation
• Track connections per server
• Handle connection reuse safely
• Severity: High - service disruption
• Complexity: High - connection pooling