feat: add self-signed SSL certificate support for Nextcloud connections

Add NEXTCLOUD_VERIFY_SSL and NEXTCLOUD_CA_BUNDLE env vars to configure
TLS certificate verification for all outbound Nextcloud connections.
Centralizes SSL config via a new HTTP client factory (http.py) used by
all 27 Nextcloud-bound call sites, including API clients, OIDC endpoints,
OAuth flows, and health checks.

Closes #560

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

env.sample

@@ -217,6 +217,19 @@ NEXTCLOUD_PASSWORD=
 #CUSTOM_PROCESSOR_TIMEOUT=60
 #CUSTOM_PROCESSOR_TYPES=application/pdf,image/jpeg,image/png
 
+# ===== SSL/TLS =====
+# For Nextcloud behind reverse proxies with self-signed or private CA certificates
+#
+# Disable TLS certificate verification (insecure, development only):
+#NEXTCLOUD_VERIFY_SSL=false
+#
+# Use a custom CA bundle (path to PEM file):
+#NEXTCLOUD_CA_BUNDLE=/etc/ssl/certs/my-ca.pem
+#
+# Docker example: mount the CA bundle as a volume
+#   docker run -v /path/to/ca.pem:/etc/ssl/certs/my-ca.pem:ro \
+#     -e NEXTCLOUD_CA_BUNDLE=/etc/ssl/certs/my-ca.pem ...
+
 # ===== SECURITY & ADVANCED =====
 # Cookie security (browser UI)
 # Auto-detects from NEXTCLOUD_HOST protocol if not set