Server-side savegames: Rust service with a shared savegame list in the menu, fallback to files

- server/: small axum service storing savegames as files (1 MB each, 500 MB total, rate limit)
- docker compose: second container with a volume; nginx forwards /api/
- JS plugin: asynchronous HTTP requests polled by the game
- Save/Load open a list page (name field, new, overwrite, load, delete with confirmation)
- Without a reachable service the game keeps using savegame files and the browser autosave

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "dungeoncraft-saves"
+version = "0.1.0"
+edition = "2021"
+license = "MIT"
+description = "Savegame storage service for DungeonCraft"
+
+[dependencies]
+axum = "0.8"
+tokio = { version = "1", features = ["rt-multi-thread", "macros", "net", "fs", "sync", "time"] }
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+
+[profile.release]
+opt-level = 3
+lto = true