feat(01-01): Backend FastAPI skeleton + Docker build config

- FastAPI app with SPAStaticFiles catch-all (root_path NOT in constructor)
- config.py with DATA_PATH, PROMPTS_PATH, OUTPUTS_PATH, CAMPAIGNS_PATH, CONFIG_PATH
- Startup lifespan creates data directories automatically
- Health endpoint: GET /api/health -> {"status": "ok"}
- Dockerfile multi-stage: node:22-slim builds React, python:3.12-slim serves API+SPA
- --root-path /postgenerator set in Uvicorn CMD only (avoids Pitfall #4)
- docker-compose.yml: lab-postgenerator-app, proxy_net, named volume for data persistence
- requirements.txt with pinned versions: fastapi[standard]==0.135.1, anthropic==0.84.0

backend/config.py

@@ -0,0 +1,53 @@
+"""Centralized configuration and path constants for PostGenerator."""
+
+import os
+from pathlib import Path
+from dataclasses import dataclass
+
+
+# ---------------------------------------------------------------------------
+# Path constants (read from env, defaulting to ./data for local dev)
+# ---------------------------------------------------------------------------
+
+DATA_PATH = Path(os.getenv("DATA_PATH", "./data"))
+PROMPTS_PATH = DATA_PATH / "prompts"
+OUTPUTS_PATH = DATA_PATH / "outputs"
+CAMPAIGNS_PATH = DATA_PATH / "campaigns"
+CONFIG_PATH = DATA_PATH / "config"
+
+
+# ---------------------------------------------------------------------------
+# App settings
+# ---------------------------------------------------------------------------
+
+@dataclass
+class Settings:
+    anthropic_api_key: str
+    llm_model: str
+    data_path: Path
+    prompts_path: Path
+    outputs_path: Path
+    campaigns_path: Path
+    config_path: Path
+
+
+def get_settings() -> Settings:
+    """Load settings from environment variables."""
+    api_key = os.getenv("ANTHROPIC_API_KEY", "")
+    if not api_key:
+        import warnings
+        warnings.warn(
+            "ANTHROPIC_API_KEY not set — generation endpoints will fail",
+            RuntimeWarning,
+            stacklevel=2,
+        )
+
+    return Settings(
+        anthropic_api_key=api_key,
+        llm_model=os.getenv("LLM_MODEL", "claude-sonnet-4-5"),
+        data_path=DATA_PATH,
+        prompts_path=PROMPTS_PATH,
+        outputs_path=OUTPUTS_PATH,
+        campaigns_path=CAMPAIGNS_PATH,
+        config_path=CONFIG_PATH,
+    )