feat: Phase 3 — web dashboard, testing, and CI/CD

- Remove multi-marketplace from Phase 3
- Add FastAPI web UI on port 8766 with basic auth
- Add 6 Jinja2 templates (dashboard, keywords, users, ads, stats)
- Add pytest test suite (45 tests, 49% coverage)
- Add GitHub Actions CI/CD workflow
- Update docker-compose.yml to expose web UI port
- Update Dockerfile to include tests

worker/Dockerfile

@@ -5,8 +5,9 @@ WORKDIR /app
 COPY requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt
 
-# ── Application code + migrations ───────────────
+# ── Application code + migrations + tests + templates ──
 COPY src/ .
+COPY tests/ tests/
 
 # Make entrypoint executable
 RUN chmod +x entrypoint.sh