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superseded docs under docs/.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.

docs/README.md

@@ -51,7 +51,7 @@ graphify-out/
 | Application type | Digital signage content/playlist/player management |
 | Graph size | **631 nodes · 1162 edges · 41 communities** |
 | Blueprints | 7 active (`main`, `auth`, `admin`, `players`, `content`, `playlist`, `api`) |
-| Database tables | 10 (`user`, `player`, `player_edit`, `player_feedback`, `player_user`, `content`, `group`, `playlist`, `server_log`, `https_config`) |
+| Database tables | 9 (`user`, `player`, `player_edit`, `player_feedback`, `player_user`, `content`, `playlist`, `server_log`, `https_config`) |
 | Reverse proxy | Caddy 2 (automatic HTTPS / Let's Encrypt) |
 | Deployment | Docker Compose (app + Caddy) + remote SSH player provisioning |
 | Key externals | LibreOffice, Poppler (pdf2image), FFmpeg, sshpass/rsync |
@@ -100,7 +100,6 @@ erDiagram
     content ||--o{ player_edit : "cascade"
     content ||--o{ player_feedback : ""
     playlist ||--o{ content : "playlist_content (M2M)"
-    content }o--o{ group : "group_content (M2M)"
     player_user ||--o{ player_edit : "user_code"
     https_config ||--|| https_config : "single row config"
 ```