fix: IPv4 preference + DNS cache fallback for stable POP3/IMAP connections

- Add DNS_CACHE_FALLBACK_ENABLED setting (default: true)
- Add in-process DNS cache (_dns_cache, _dns_cache_lock, _get/_set_cached_ipv4)
- Add _resolve_ipv4 (async) and _resolve_ipv4_sync: AF_INET lookup, cache on
  success, return cached IP on EAI_AGAIN when fallback enabled
- Add _IMAP4SSLwithSNI: connects to pre-resolved IPv4 but uses original
  hostname for TLS SNI/cert verification
- Add _POP3SSLWithIPv4Pref / _POP3WithIPv4Pref: override _create_socket to
  use pre-resolved IPv4 while keeping original host for POP3_SSL SNI
- Add _make_pop3_conn factory and _make_imap_client async factory
- Wire IPv4/cache helpers into all 6 connection call-sites (IMAP x3, POP3 x3)
- Distinguish transient DNS (EAI_AGAIN) from permanent in _format_connection_error
- Replace asyncio.get_event_loop() with get_running_loop() in 4 async sites
- All 75 existing unit tests pass unchanged

Agent-Logs-Url: https://github.com/christianlouis/InboxConverge/sessions/be0a3bbd-1af1-408e-b918-84753723154b

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

docs/TODO.md

@@ -4,6 +4,20 @@ Comprehensive task breakdown for repository improvements and production readines
 
 ## ✅ Recently Completed
 
+- [x] **IMAP/POP3 connection stability — IPv4 preference + DNS cache**: All
+  POP3 and IMAP connections now prefer IPv4 by pre-resolving hostnames with
+  `AF_INET` before connecting.  This eliminates `ENETUNREACH` / `Network is
+  unreachable` and sporadic TLS timeouts caused by Docker containers attempting
+  IPv6 connections to servers that block IPv6.  Successfully resolved IPv4
+  addresses are cached in-process; when `DNS_CACHE_FALLBACK_ENABLED=true`
+  (default) the cached address is used as a fallback if DNS subsequently fails
+  (`EAI_AGAIN`), keeping mail delivery alive through transient resolver outages.
+  Added `_resolve_ipv4` / `_resolve_ipv4_sync`, `_IMAP4SSLwithSNI`,
+  `_POP3SSLWithIPv4Pref`, `_POP3WithIPv4Pref`, `_make_pop3_conn`, and
+  `_make_imap_client` helpers.  Transient DNS errors now report "Temporary DNS
+  failure" rather than "check that the server address is correct".  Replaced
+  four deprecated `asyncio.get_event_loop()` calls with `get_running_loop()`.
+
 - [x] **IMAP/POP3 diagnostics — Step 1: Friendly error messages**: Added
   `_format_connection_error()` helper that converts raw OS/socket/SSL/POP3/IMAP
   exceptions into human-readable sentences with host:port context.  Applied at