feat: connection pooling via urllib3 + batch OG fetching

Replace per-request SOCKS5+TLS handshakes with urllib3 SOCKSProxyManager
connection pool (20 pools, 4 conns/host). Batch _fetch_og calls via
ThreadPoolExecutor to parallelize OG tag enrichment in alert polling.
Cache flaskpaste SSL context at module level.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pyproject.toml

@@ -11,6 +11,7 @@ license = "MIT"
 dependencies = [
     "maxminddb>=2.0",
     "PySocks>=1.7.1",
+    "urllib3[socks]>=2.0",
 ]
 
 [project.scripts]