refactor(storage): new storage file structure

- new directory structure containing manifest.json and one json file per provider, by default.
- the manifest.json file can specify a filepath for each vpn provider
- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch
- `STORAGE_SERVERS_DIRECTORY_PATH` replaces `STORAGE_FILEPATH` (which is now a migration source only). It sets the directory where server manifest and per-provider JSON files are stored (default: `/gluetun/servers/`).
- First-run migration: On startup, gluetun checks for the old /gluetun/servers.json file; if found and no new manifest exists, it automatically migrates all data to /gluetun/servers/ directory structure
- Silent fallback: If legacy file isn't found, uses the new directory path normally
- Legacy cleanup: After successful migration, attempts to remove the old fat JSON file (logs warning only if removal fails, e.g., read-only bind mounts)

Co-authored-by: Copilot <copilot@github.com>

Dockerfile

@@ -273,7 +273,7 @@ ENV VPN_SERVICE_PROVIDER=pia \
     PUBLICIP_API=ipinfo,ifconfigco,ip2location,cloudflare \
     PUBLICIP_API_TOKEN= \
     # Storage
-    STORAGE_FILEPATH=/gluetun/servers.json \
+    STORAGE_SERVERS_DIRECTORY_PATH=/gluetun/servers/ \
     # Pprof
     PPROF_ENABLED=no \
     PPROF_BLOCK_PROFILE_RATE=0 \