Initial nix-ota implementation

Self-hostable OTA update system for NixOS fleets: a control server,
device agent, publisher CLI, and NixOS modules that ship prebuilt
system closures from a binary cache to devices that don't have the
flake.

- crates/common: signed manifest types (ed25519), store-path validator
- crates/server: axum + sqlite + HTMX dashboard, channel/device API
- crates/agent: poll, verify signature + revision, nix copy, switch,
  health check, magic-rollback on failure
- crates/publisher: keygen + sign + publish CLI for operators/CI
- nix/modules: NixOS modules for server and agent
- nix/tests/ota.nix: end-to-end VM test exercising publish A -> B ->
  broken C -> rollback to B (passes)

The control server never holds the signing key; manifests are signed
offline and verified against a pinned public key on each device.

Cargo.toml

@@ -0,0 +1,33 @@
+[workspace]
+resolver = "2"
+members = ["crates/common", "crates/server", "crates/agent", "crates/publisher"]
+
+[workspace.package]
+version = "0.1.0"
+edition = "2021"
+license = "MIT OR Apache-2.0"
+repository = "https://linus.dyrehytten.dk/max/nix-ota"
+
+[workspace.dependencies]
+anyhow = "1"
+thiserror = "1"
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+tokio = { version = "1", features = ["full"] }
+tracing = "0.1"
+tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+clap = { version = "4", features = ["derive", "env"] }
+reqwest = { version = "0.12", features = ["json", "rustls-tls"], default-features = false }
+ed25519-dalek = { version = "2", features = ["rand_core", "pkcs8", "pem"] }
+rand = "0.8"
+base64 = "0.22"
+hex = "0.4"
+time = { version = "0.3", features = ["serde", "formatting", "parsing", "macros"] }
+axum = "0.7"
+tower = "0.5"
+tower-http = { version = "0.6", features = ["trace", "fs"] }
+sqlx = { version = "0.8", features = ["runtime-tokio-rustls", "sqlite", "time", "migrate"] }
+askama = { version = "0.12", features = ["with-axum"] }
+askama_axum = "0.4"
+ulid = { version = "1", features = ["serde"] }
+sha2 = "0.10"