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- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch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>

go.mod

@@ -54,6 +54,7 @@ require (
 	github.com/prometheus/client_model v0.6.1 // indirect
 	github.com/prometheus/common v0.60.1 // indirect
 	github.com/prometheus/procfs v0.15.1 // indirect
+	github.com/qdm12/gluetun-servers v0.1.0
 	github.com/qdm12/goservices v0.1.1-0.20251104135713-6bee97bd4978 // indirect
 	github.com/riobard/go-bloom v0.0.0-20200614022211-cdc8013cb5b3 // indirect
 	golang.org/x/crypto v0.48.0 // indirect
@@ -66,3 +67,5 @@ require (
 	kernel.org/pub/linux/libs/security/libcap/cap v1.2.70 // indirect
 	kernel.org/pub/linux/libs/security/libcap/psx v1.2.70 // indirect
 )
+
+replace github.com/qdm12/gluetun-servers => ./gluetun-servers //nolint:gomoddirectives