chore: consolidate runtime data layout under single data/ root directory

Migrate all runtime artifacts (database, uploads, VAPID keys, locks, bot state, container workspaces, zip staging) from scattered locations (`var/`, `devplacepy/static/uploads/`) into a unified `data/` directory. Update `config.py` as single source of truth with `DATA_PATHS` registry and `ensure_data_dirs()`, add `devplace migrate-data` CLI command with CRC verification and idempotent relocation, adjust `.dockerignore`, `.env.example`, `.gitignore`, `Dockerfile`, `Makefile`, `README.md`, `AGENTS.md`, `CLAUDE.md`, and all import paths in `attachments.py` to reference `config.UPLOADS_DIR`/`ATTACHMENTS_DIR` instead of computing from `STATIC_DIR`.

nginx/nginx.conf.template

@@ -32,7 +32,7 @@ server {
     gzip_proxied any;
 
     location /static/uploads/ {
-        alias /app/static/uploads/;
+        alias /data/uploads/;
         add_header X-Content-Type-Options nosniff;
         add_header Content-Disposition $upload_disposition;
         add_header Cache-Control "public, max-age=604800";