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superseded docs under docs/.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/content.py

@@ -38,8 +38,6 @@ class Content(db.Model):
     # Relationships - many-to-many with playlists
     playlists = db.relationship('Playlist', secondary='playlist_content', 
                                back_populates='contents', lazy='dynamic')
-    groups = db.relationship('Group', secondary='group_content', 
-                            back_populates='contents', lazy='dynamic')
     
     def __repr__(self) -> str:
         """String representation of Content."""
@@ -52,11 +50,6 @@ class Content(db.Model):
             return round(self.file_size / (1024 * 1024), 2)
         return 0.0
     
-    @property
-    def group_count(self) -> int:
-        """Get number of groups containing this content."""
-        return self.groups.count()
-
     @property
     def original_display_name(self) -> str:
         """Name of the original (unedited) file for display purposes."""