feat(media): multimodal attachments — image/video inlining per model capabilities

Adds media.rs for per-turn attachment routing, message_builder
partitioning by resolved model capabilities, controller endpoints
for uploads, and server routing for /data/* behind session auth.

See CLAUDE.md §Multimodal attachments for full design.

src/frontend/api/mod.rs

@@ -210,6 +210,10 @@ impl ApiError {
     pub fn forbidden(msg: impl Into<String>) -> Self {
         Self { status: StatusCode::FORBIDDEN, message: msg.into() }
     }
+
+    pub fn payload_too_large(msg: impl Into<String>) -> Self {
+        Self { status: StatusCode::PAYLOAD_TOO_LARGE, message: msg.into() }
+    }
 }
 
 /// Resolves the authenticated caller's per-user runtime context, or `401` when the