feat: migrate imphnen-backend-hackathon into workspace as imphnen-hackathon crate

Consolidates the standalone hackathon backend (16 crates) into a single
imphnen-hackathon crate following the existing clean architecture patterns.
All endpoints are exposed under /v1/hackathon/ via the gateway.

Features migrated:
- Auth: Supabase-based signup/login/GitHub OAuth/password reset (own JWT)
- Users: profile management with team listing
- Teams: CRUD with city validation, deadline enforcement, invite system
- Invitations: team member invitations with accept/reject flow
- Join Requests: team join request workflow
- Chat: team messaging with author/leader delete permissions
- Submissions: project submission lifecycle (draft→pending→submitted)
- Storage: Supabase Storage file upload endpoints
- Certificates: public user certificate data endpoint
- Winners: public winners listing
- Admin: admin-only CRUD for all entities

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

imphnen-hackathon/src/join_requests/domain/service.rs

@@ -0,0 +1,29 @@
+use async_trait::async_trait;
+use uuid::Uuid;
+use imphnen_utils::errors::AppError;
+use super::entity::*;
+
+#[async_trait]
+pub trait JoinRequestService: Send + Sync {
+    async fn create_join_request(
+        &self,
+        team_id: Uuid,
+        user_id: Uuid,
+        input: CreateJoinRequestInput,
+    ) -> Result<JoinRequestWithDetails, AppError>;
+
+    async fn get_my_join_requests(&self, user_id: Uuid) -> Result<Vec<JoinRequestWithDetails>, AppError>;
+
+    async fn get_team_join_requests(
+        &self,
+        team_id: Uuid,
+        user_id: Uuid,
+    ) -> Result<Vec<JoinRequestWithDetails>, AppError>;
+
+    async fn respond_to_join_request(
+        &self,
+        request_id: Uuid,
+        user_id: Uuid,
+        accept: bool,
+    ) -> Result<(), AppError>;
+}