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/submissions/domain/entity.rs

@@ -0,0 +1,39 @@
+use uuid::Uuid;
+use chrono::{DateTime, Utc};
+
+#[derive(Debug, Clone)]
+pub struct SubmissionEntity {
+    pub id: Uuid,
+    pub team_id: Uuid,
+    pub project_name: String,
+    pub description: String,
+    pub repository_url: String,
+    pub demo_url: Option<String>,
+    pub presentation_url: Option<String>,
+    pub screenshots: Option<Vec<String>>,
+    pub status: String,
+    pub submitted_at: Option<DateTime<Utc>>,
+    pub submitted_by: Uuid,
+    pub created_at: Option<DateTime<Utc>>,
+    pub updated_at: Option<DateTime<Utc>>,
+}
+
+#[derive(Debug, Default)]
+pub struct CreateSubmissionInput {
+    pub project_name: String,
+    pub description: String,
+    pub repository_url: String,
+    pub demo_url: Option<String>,
+    pub presentation_url: Option<String>,
+    pub screenshots: Option<Vec<String>>,
+}
+
+#[derive(Debug, Default)]
+pub struct UpdateSubmissionInput {
+    pub project_name: Option<String>,
+    pub description: Option<String>,
+    pub repository_url: Option<String>,
+    pub demo_url: Option<String>,
+    pub presentation_url: Option<String>,
+    pub screenshots: Option<Vec<String>>,
+}