feat!: backend-only architecture, remove frontend/nginx (v1.2.0)

- Remove frontend/ and nginx/ from core (each customer owns their frontend)
- Update docker-compose.yml to backend-only (postgres + redis + backend)
- Update docker-compose.prod.yml to backend-only
- Add CLAUDE.md with dev guidelines
- Update README.md with new architecture diagram

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -0,0 +1,33 @@
+# EMS Core Development Guidelines
+
+## Overview
+This is the shared core library for all EMS customer projects. Changes here affect ALL customer deployments.
+
+## Golden Rules
+- All changes must go through Pull Request with code review
+- Follow Conventional Commits: `<type>(<scope>): <description>`
+- Types: feat, fix, refactor, docs, test, chore, style, perf
+- Scopes: api, models, services, collector, hooks, tasks, config
+
+## Architecture
+- `backend/app/api/v1/` — API endpoint modules
+- `backend/app/collectors/` — Device data collectors (Sungrow, Modbus, MQTT, HTTP)
+- `backend/app/hooks/` — Customer plugin system (base + loader)
+- `backend/app/models/` — SQLAlchemy ORM models
+- `backend/app/services/` — Business logic layer
+- `backend/app/tasks/` — Background tasks (Celery + APScheduler)
+- `backend/alembic/` — Database migrations
+- **Note:** Frontend is NOT included in ems-core. Each customer repo owns its own frontend.
+
+## Database Migrations
+- Every model change requires an Alembic migration
+- Test migrations before deploying: `alembic upgrade head`
+- Never manually edit `alembic_version` table
+
+## Testing
+- Run tests: `cd backend && pytest`
+- Minimum test coverage for new features
+
+## Version
+- Follow SemVer (MAJOR.MINOR.PATCH)
+- Current: see VERSION file