feat: relay identity persistence + Linux build script

Relay identity:
- Stored in ~/.wzp/relay-identity (hex-encoded 32-byte seed)
- Generated on first run, reused on restart
- Fingerprint stays consistent across relay restarts

Linux build script (scripts/build-linux-notify.sh):
- Fire and forget: Hetzner VM → build all binaries → upload to rustypaste → ntfy notify → destroy VM
- Builds: wzp-relay, wzp-client, wzp-client-audio, wzp-web, wzp-bench
- Packages as tar.gz, uploads to rustypaste
- --keep flag to preserve VM

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -847,6 +847,27 @@ dependencies = [
  "subtle",
 ]
 
+[[package]]
+name = "dirs"
+version = "6.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c3e8aa94d75141228480295a7d0e7feb620b1a5ad9f12bc40be62411e38cce4e"
+dependencies = [
+ "dirs-sys",
+]
+
+[[package]]
+name = "dirs-sys"
+version = "0.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e01a3366d27ee9890022452ee61b2b63a67e6f13f58900b651ff5665f0bb1fab"
+dependencies = [
+ "libc",
+ "option-ext",
+ "redox_users",
+ "windows-sys 0.61.2",
+]
+
 [[package]]
 name = "displaydoc"
 version = "0.2.5"
@@ -1760,6 +1781,15 @@ version = "0.2.16"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b6d2cec3eae94f9f509c767b45932f1ada8350c4bdb85af2fcab4a3c14807981"
 
+[[package]]
+name = "libredox"
+version = "0.1.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7ddbf48fd451246b1f8c2610bd3b4ac0cc6e149d89832867093ab69a17194f08"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "linux-raw-sys"
 version = "0.12.1"
@@ -2089,6 +2119,12 @@ dependencies = [
  "vcpkg",
 ]
 
+[[package]]
+name = "option-ext"
+version = "0.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "04744f49eae99ab78e0d5c0b603ab218f515ea8cfe5a456d7629ad883a3b6e7d"
+
 [[package]]
 name = "os_str_bytes"
 version = "6.6.1"
@@ -2429,6 +2465,17 @@ dependencies = [
  "bitflags 2.11.0",
 ]
 
+[[package]]
+name = "redox_users"
+version = "0.5.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a4e608c6638b9c18977b00b475ac1f28d14e84b27d8d42f70e0bf1e3dec127ac"
+dependencies = [
+ "getrandom 0.2.17",
+ "libredox",
+ "thiserror 2.0.18",
+]
+
 [[package]]
 name = "regex"
 version = "1.12.3"
@@ -4323,6 +4370,7 @@ dependencies = [
  "async-trait",
  "axum 0.7.9",
  "bytes",
+ "dirs",
  "futures-util",
  "prometheus",
  "quinn",