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>

Containerfile

@@ -2,7 +2,8 @@ FROM python:3.13-alpine
 
 WORKDIR /app
 
-RUN pip install --no-cache-dir maxminddb>=2.0 PySocks>=1.7.1
+COPY requirements.txt .
+RUN pip install --no-cache-dir -r requirements.txt
 
 ENV PYTHONPATH=/app/src
 ENV PYTHONUNBUFFERED=1