feat: add Quart app factory, config loader, and /api/health endpoint

Config reads from env vars (12-factor); .env.example documents defaults.
Health endpoint is liveness-only (no DB/Redis touch). Test added for CI.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/config.py

@@ -0,0 +1,52 @@
+"""Application configuration loaded from environment variables."""
+
+import os
+from dataclasses import dataclass
+from functools import lru_cache
+
+
+@dataclass(frozen=True)
+class Config:
+    db_user: str
+    db_password: str
+    db_host: str
+    db_port: int
+    db_name: str
+
+    celery_broker_url: str
+    celery_result_backend: str
+
+    secret_key: str
+    extension_api_key: str  # used by the Firefox extension; lands in FC-3 but read here
+    log_level: str
+
+    @property
+    def database_url(self) -> str:
+        return (
+            f"postgresql+asyncpg://{self.db_user}:{self.db_password}"
+            f"@{self.db_host}:{self.db_port}/{self.db_name}"
+        )
+
+    @property
+    def database_url_sync(self) -> str:
+        # Alembic uses sync driver
+        return (
+            f"postgresql+psycopg://{self.db_user}:{self.db_password}"
+            f"@{self.db_host}:{self.db_port}/{self.db_name}"
+        )
+
+
+@lru_cache(maxsize=1)
+def get_config() -> Config:
+    return Config(
+        db_user=os.environ.get("DB_USER", "fabledcurator"),
+        db_password=os.environ["DB_PASSWORD"],
+        db_host=os.environ.get("DB_HOST", "postgres"),
+        db_port=int(os.environ.get("DB_PORT", "5432")),
+        db_name=os.environ.get("DB_NAME", "fabledcurator"),
+        celery_broker_url=os.environ.get("CELERY_BROKER_URL", "redis://redis:6379/0"),
+        celery_result_backend=os.environ.get("CELERY_RESULT_BACKEND", "redis://redis:6379/0"),
+        secret_key=os.environ["SECRET_KEY"],
+        extension_api_key=os.environ.get("EXTENSION_API_KEY", ""),
+        log_level=os.environ.get("LOG_LEVEL", "INFO"),
+    )