feat: Site Announcements routes migrated to Rust

- New SiteAnnouncement model (site_announcement.rs)
- Route handlers: list (with siteId/platform/read/status filters),
  mark read, mark all read, delete all
- In-memory status filtering for active/expired/dismissed (same as Node.js)
- Uses existing site_announcements table from migration
- 18/18 tests pass

boosapi-rs/src/db/models/mod.rs

@@ -6,5 +6,6 @@ pub mod event;
 pub mod route_channel;
 pub mod setting;
 pub mod site;
+pub mod site_announcement;
 pub mod token_route;
 pub mod user;

boosapi-rs/src/db/models/site_announcement.rs

@@ -0,0 +1,24 @@
+use serde::{Deserialize, Serialize};
+use sqlx::FromRow;
+
+/// Site-level announcements (notifications from upstream providers).
+#[derive(Debug, Clone, Serialize, Deserialize, FromRow)]
+pub struct SiteAnnouncement {
+    pub id: i64,
+    pub site_id: i64,
+    pub platform: String,
+    pub source_key: String,
+    pub title: String,
+    pub content: String,
+    pub level: Option<String>,
+    pub source_url: Option<String>,
+    pub starts_at: Option<String>,
+    pub ends_at: Option<String>,
+    pub upstream_created_at: Option<String>,
+    pub upstream_updated_at: Option<String>,
+    pub first_seen_at: Option<String>,
+    pub last_seen_at: Option<String>,
+    pub read_at: Option<String>,
+    pub dismissed_at: Option<String>,
+    pub raw_payload: Option<String>,
+}