Sanitize codebase, reorganize docs, and add missing deploy files

Remove dead code identified in docs/SANITIZATION-REVIEW.md:
- app/blueprints/content_old.py and app/blueprints/playlist.py
- app/models/group.py, app/utils/nginx_config_reader.py
- orphaned templates (content_list, edit_content, upload_content,
  player_page) and the related group/Template references

Result: 6 blueprints, 82 routes, no dead modules or orphan templates.

Add files that deploy.sh and docker-entrypoint.sh already require but
which were never tracked:
- https_manager.py       (referenced by deploy.sh, migrate_network.sh,
                          docker-entrypoint.sh)
- Caddyfile.example      (seeded by deploy.sh; its absence aborts deploy)

Relocate generated Graphify artifacts from graphify-out/ to
docs/graphify-out/ (110 files, no content change) and archive the
superseded docs under docs/.

Ignore hygiene:
- ignore ad-hoc .env backups (.env.bak*) — they contain live secrets
- keep the pre-sanitization snapshots (docs/legacy code/,
  docs/old_code_documentation/) on disk but out of the repo

Fix .env.example: drop a duplicated config block, genericize the
hardcoded host IP, and document HOSTNAME_INTERNAL.

Caddyfile.example

@@ -0,0 +1,35 @@
+{
+    # Caddy admin API — used by DigiServer to reload config after HTTPS is enabled
+    admin 0.0.0.0:2019
+}
+
+# Default: serve the app on HTTP port 80
+# Once HTTPS is configured via Admin → HTTPS Config, Caddy will reload
+# this file and start provisioning a Let's Encrypt certificate automatically.
+:80 {
+    reverse_proxy digiserver-app:5000 {
+        header_up Host {host}
+        header_up X-Real-IP {remote_host}
+        header_up X-Forwarded-Proto {scheme}
+        transport http {
+            read_timeout 300s
+            write_timeout 300s
+        }
+    }
+
+    request_body {
+        max_size 2GB
+    }
+
+    encode gzip
+
+    header {
+        X-Frame-Options "SAMEORIGIN"
+        X-Content-Type-Options "nosniff"
+        X-XSS-Protection "1; mode=block"
+    }
+
+    log {
+        output file /var/log/caddy/access.log
+    }
+}