feat: Add case closing and document viewing routes

Adds API endpoints for closing a case, which triggers an analysis job,
and for viewing the generated document.

This also includes:
- New `AppError` variants: `Conflict` and `ServiceUnavailable`.
- Helper functions for file handling, date parsing, and document
  searching.
- Unit tests for helper functions.

server/src/error.rs

@@ -6,6 +6,11 @@ pub enum AppError {
     Unauthorized,
     BadRequest(String),
     NotFound(String),
+    /// 409 — request is valid but conflicts with current state (e.g. an
+    /// analysis is already running for this case).
+    Conflict(String),
+    /// 503 — the feature is unavailable right now (e.g. no LLM configured).
+    ServiceUnavailable(String),
     Internal(String),
     /// 302 redirect. Used by web auth to send unauthenticated users to login.
     Redirect(String),
@@ -25,6 +30,8 @@ impl IntoResponse for AppError {
             Self::Unauthorized => (StatusCode::UNAUTHORIZED, "Unauthorized".to_owned()),
             Self::BadRequest(msg) => (StatusCode::BAD_REQUEST, msg.clone()),
             Self::NotFound(msg) => (StatusCode::NOT_FOUND, msg.clone()),
+            Self::Conflict(msg) => (StatusCode::CONFLICT, msg.clone()),
+            Self::ServiceUnavailable(msg) => (StatusCode::SERVICE_UNAVAILABLE, msg.clone()),
             Self::Internal(msg) => (StatusCode::INTERNAL_SERVER_ERROR, msg.clone()),
             Self::Redirect(_) => unreachable!(),
         };