feat(gotify): Phase 3 C1 — Gotify REST API client

- src/gotify/api.rs: blocking reqwest 封装（§14）；health / applications
  / messages 三个方法；Bearer token 用 HeaderValue::set_sensitive
  标记，日志绝不泄露（§20）；10s per-request timeout
- src/gotify/model.rs: GotifyHealth / GotifyApplication / GotifyMessage
  DTO 镜像官方 wire schema；defaultPriority 加 rename（camelCase）
- src/gotify/mod.rs: re-export
- src/error.rs: 加 AppError::Gotify(String)
- Cargo.toml: reqwest 0.12 (blocking + json + rustls-tls) 从注释升级为真
  依赖（AGENTS.md §44 模板里早就列了，Phase 1 当时推迟）
- tests/gotify_api.rs: 6 个 JSON 解析测试

避坑：
- reqwest 0.12 默认 log Bearer token；要 set_sensitive(true) 才行
- reqwest::Error 转 AppError 不能用返回 impl Fn 的 helper（path 是
  &str 不是 'static）—— 改成每个调用点 inline closure
- Gotify 的 defaultPriority 是 camelCase，其他字段都是单字；
  用 #[serde(rename = "defaultPriority")] 单点 rename 而不是全局
  rename_all = "camelCase"（因为 Rust 端字段名是 snake_case）

验证：cargo build / build --release 都 exit 0，0 warning
- cargo test --test gotify_api: 6 passed; 0 failed
- debug   27.88 MB, subsystem = WINDOWS_CUI
- release  8.10 MB, subsystem = WINDOWS_GUI（reqwest 加进 release
  LTO 后体积几乎没变——rustls 是大头但被 strip + LTO 吃掉了）

Cargo.toml

@@ -38,6 +38,9 @@ thiserror = "2"
 serde     = { version = "1", features = ["derive"] }
 serde_json = "1"
 
+# --- Networking (Phase 3 Gotify HTTP) ---
+reqwest = { version = "0.12", default-features = false, features = ["blocking", "json", "rustls-tls"] }
+
 # Slint's `.slint` files are compiled by `build.rs`; see build.rs.
 [build-dependencies]
 slint-build = "1"