feat(gitea): API routes + per-user client + repo contents

GiteaClient:
- __init__(user_token=None) — per-user OAuth token ou admin token
- get_repo_contents(owner, repo, path) — lazy: un niveau à la fois
- get_file_content(owner, repo, path) — base64 décodé
- create_or_update_file(...) — PUT avec sha pour update
- delete_file(owner, repo, path, sha, message)
- _invalidate_tree_cache() — invalidation après write

Routes (/api/gitea):
- GET /orgs — liste des organisations
- GET /projects?org=x — repos user ou org
- GET /projects/{owner}/{repo}/tree?path=x — arborescence lazy
- GET /projects/{owner}/{repo}/file?path=x — contenu fichier
- PUT /projects/{owner}/{repo}/file — save + commit
- DELETE /projects/{owner}/{repo}/file?path=x&sha=x — delete
- GET /projects/{owner}/{repo}/labels — labels → tags

Helper: get_user_gitea_client(request) → GiteaClient ou None

app/main.py

@@ -13,6 +13,7 @@ from app.config import settings
 from app.db import init_db
 from app.middleware.csrf import CSRFMiddleware
 from app.routers import dashboard, board, notes, api, auth, webhooks, collections, my_tasks, workspace, public_api, admin
+from app.routers.gitea import router as gitea_router
 from app.services.gitea_client import gitea
 from app.services.webhook_outbound import init_webhook_tables
 
@@ -59,6 +60,7 @@ app.include_router(collections.router)
 app.include_router(my_tasks.router)
 app.include_router(workspace.router)
 app.include_router(admin.router)
+app.include_router(gitea_router)
 app.include_router(public_api.router)
 
 app.mount("/static", StaticFiles(directory="static"), name="static")