Add modular app structure for L1 server refactoring

Phase 2 of the full modernization:
- App factory pattern with create_app()
- Settings via dataclass with env vars
- Dependency injection container
- Router stubs for auth, storage, api, recipes, cache, runs
- Service layer stubs for run, recipe, cache
- Repository layer placeholder

Routes are stubs that import from legacy server.py during migration.
Next: Migrate each router fully with templates.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/repositories/__init__.py

@@ -0,0 +1,10 @@
+"""
+L1 Server Repositories.
+
+Data access layer for persistence operations.
+"""
+
+# TODO: Implement repositories
+# - RunRepository - Redis-backed run storage
+# - RecipeRepository - Redis-backed recipe storage
+# - CacheRepository - Filesystem + PostgreSQL cache metadata