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`.

docker-compose.yml

@@ -39,6 +39,7 @@ services:
       NGINX_MAX_BODY_SIZE: "${NGINX_MAX_BODY_SIZE:-50m}"
     volumes:
       - ./devplacepy/static:/app/static:ro
+      - ${DEVPLACE_DATA_DIR:-./data}/uploads:/data/uploads:ro
     depends_on:
       app:
         condition: service_healthy