step 1 complete: runnable FastAPI skeleton

- Full directory structure with all __init__.py stubs
- requirements.txt with all pinned dependencies
- app/config/settings.py (BaseSettings, env-based)
- app/main.py (CORS, lifespan, /api/v1/health)
- Dockerfile (multi-stage, Python 3.12-slim, non-root user)
- docker-compose.yml (app + postgres:16 with healthcheck)
- .env.example
- BACKEND_PLAN.md: mark step 1 done, add one-step-at-a-time rule

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/config/settings.py

@@ -0,0 +1,31 @@
+from typing import Literal
+from pydantic_settings import BaseSettings
+
+
+class Settings(BaseSettings):
+    DATABASE_URL: str = "postgresql+asyncpg://postgres:postgres@localhost:5432/adiuva"
+    JWT_SECRET: str = "change-me-in-production"
+    JWT_ALGORITHM: str = "HS256"
+    JWT_ACCESS_TOKEN_EXPIRE_MINUTES: int = 30
+    JWT_REFRESH_TOKEN_EXPIRE_DAYS: int = 30
+
+    STRIPE_SECRET_KEY: str = ""
+    STRIPE_WEBHOOK_SECRET: str = ""
+
+    S3_BUCKET: str = ""
+    S3_REGION: str = "us-east-1"
+    AWS_ACCESS_KEY_ID: str = ""
+    AWS_SECRET_ACCESS_KEY: str = ""
+
+    OPENAI_API_KEY: str = ""
+
+    CORS_ORIGINS: list[str] = ["app://.", "http://localhost:3000", "http://localhost:5173"]
+
+    ENV: Literal["dev", "prod"] = "dev"
+
+    class Config:
+        env_file = ".env"
+        env_file_encoding = "utf-8"
+
+
+settings = Settings()