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superseded docs under docs/.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.

app/models/__init__.py

@@ -1,7 +1,6 @@
 """Models package for digiserver-v2."""
 from app.models.user import User
 from app.models.player import Player
-from app.models.group import Group, group_content
 from app.models.playlist import Playlist, playlist_content
 from app.models.content import Content
 from app.models.server_log import ServerLog
@@ -13,7 +12,6 @@ from app.models.https_config import HTTPSConfig
 __all__ = [
     'User',
     'Player',
-    'Group',
     'Playlist',
     'Content',
     'ServerLog',
@@ -21,6 +19,5 @@ __all__ = [
     'PlayerEdit',
     'PlayerUser',
     'HTTPSConfig',
-    'group_content',
     'playlist_content',
 ]