feat: backend ingest API, Docker Compose, Ubuntu install guide

- FastAPI: POST /api/v1/events, GET /health, JSON Schema validation

- PostgreSQL models, Alembic migration, bootstrap API key

- deploy/docker-compose.yml, .env.example

- docs/install-ubuntu-24.04.md, updated work-plan (agents first)

backend/app/config.py

@@ -0,0 +1,33 @@
+from functools import lru_cache
+from pathlib import Path
+
+from pydantic_settings import BaseSettings, SettingsConfigDict
+
+ROOT = Path(__file__).resolve().parents[2]
+SCHEMA_PATH = ROOT / "schemas" / "event-schema-v1.json"
+
+
+class Settings(BaseSettings):
+    model_config = SettingsConfigDict(
+        env_file=".env",
+        env_file_encoding="utf-8",
+        extra="ignore",
+    )
+
+    database_url: str = "postgresql+psycopg2://sac:sac@localhost:5432/sac"
+    sac_public_url: str = "http://localhost:8000"
+    jwt_secret: str = "change-me-in-production"
+    jwt_algorithm: str = "HS256"
+    jwt_expire_minutes: int = 60 * 24
+
+    sac_bootstrap_api_key: str = ""
+    sac_admin_username: str = "admin"
+    sac_admin_password: str = ""
+
+    event_schema_path: str = str(SCHEMA_PATH)  # override: EVENT_SCHEMA_PATH
+    cors_origins: str = "*"
+
+
+@lru_cache
+def get_settings() -> Settings:
+    return Settings()