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,
+    )

backend/main.py

@@ -0,0 +1,78 @@
+"""PostGenerator FastAPI application.
+
+IMPORTANT: root_path is intentionally NOT set in the FastAPI() constructor.
+It is passed only via Uvicorn's --root-path flag at runtime to avoid the
+double-path bug (Pitfall #4).
+"""
+
+from contextlib import asynccontextmanager
+from pathlib import Path
+
+from fastapi import FastAPI
+from fastapi.responses import FileResponse
+from fastapi.staticfiles import StaticFiles
+
+from backend.config import CAMPAIGNS_PATH, CONFIG_PATH, OUTPUTS_PATH, PROMPTS_PATH
+
+
+# ---------------------------------------------------------------------------
+# SPA static files with catch-all fallback
+# ---------------------------------------------------------------------------
+
+class SPAStaticFiles(StaticFiles):
+    """Serve a React SPA: return index.html for any 404 so the client-side
+    router handles unknown paths instead of returning a 404 from the server."""
+
+    async def get_response(self, path: str, scope):
+        try:
+            return await super().get_response(path, scope)
+        except Exception:
+            # Fall back to index.html for client-side routing
+            return await super().get_response("index.html", scope)
+
+
+# ---------------------------------------------------------------------------
+# Startup / shutdown lifecycle
+# ---------------------------------------------------------------------------
+
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    """Create required data directories on startup if they do not exist."""
+    for directory in (PROMPTS_PATH, OUTPUTS_PATH, CAMPAIGNS_PATH, CONFIG_PATH):
+        directory.mkdir(parents=True, exist_ok=True)
+    yield
+    # Nothing to clean up on shutdown
+
+
+# ---------------------------------------------------------------------------
+# Application
+# ---------------------------------------------------------------------------
+
+# CRITICAL: Do NOT pass root_path here — use Uvicorn --root-path instead.
+app = FastAPI(
+    title="PostGenerator",
+    description="Instagram carousel post generator powered by Claude",
+    version="0.1.0",
+    lifespan=lifespan,
+    # root_path is intentionally absent — set via Uvicorn --root-path at runtime
+)
+
+
+# ---------------------------------------------------------------------------
+# API routes (must be registered BEFORE the SPA catch-all mount)
+# ---------------------------------------------------------------------------
+
+@app.get("/api/health")
+async def health() -> dict:
+    """Health check endpoint."""
+    return {"status": "ok"}
+
+
+# ---------------------------------------------------------------------------
+# SPA catch-all mount (MUST be last — catches everything not matched above)
+# ---------------------------------------------------------------------------
+
+_STATIC_DIR = Path(__file__).parent.parent / "static"
+
+if _STATIC_DIR.exists():
+    app.mount("/", SPAStaticFiles(directory=str(_STATIC_DIR), html=True), name="spa")