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/Cargo.toml

@@ -0,0 +1,25 @@
+[package]
+name = "imphnen-hackathon"
+version = "0.2.0"
+edition = "2024"
+
+[dependencies]
+imphnen-utils.workspace = true
+imphnen-libs.workspace = true
+axum.workspace = true
+axum-extra.workspace = true
+sea-orm.workspace = true
+sqlx.workspace = true
+async-trait.workspace = true
+serde.workspace = true
+serde_json.workspace = true
+utoipa.workspace = true
+chrono.workspace = true
+uuid.workspace = true
+tokio.workspace = true
+reqwest.workspace = true
+lettre.workspace = true
+base64.workspace = true
+jsonwebtoken.workspace = true
+tracing.workspace = true
+thiserror.workspace = true