feat(auth): add rate limiting to login and register endpoints

- Add slowapi rate limiter: 10 req/min for /auth/login, 5 req/min for /auth/register
- Return HTTP 429 with Retry-After header when limit is exceeded
- Add slowapi to requirements.txt
- Add 4 passing tests for rate limit behavior

Closes leeworks-agents/SPARC#9

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

SPARC/api.py

@@ -7,9 +7,13 @@ from contextlib import asynccontextmanager
 from datetime import datetime
 from typing import Annotated, List
 
-from fastapi import BackgroundTasks, Depends, FastAPI, HTTPException, Query
+from fastapi import BackgroundTasks, Depends, FastAPI, HTTPException, Query, Request
 from fastapi.middleware.cors import CORSMiddleware
+from fastapi.responses import JSONResponse
 from pydantic import BaseModel, EmailStr, Field
+from slowapi import Limiter
+from slowapi.errors import RateLimitExceeded
+from slowapi.util import get_remote_address
 
 from SPARC import config
 from SPARC.analyzer import CompanyAnalyzer
@@ -164,6 +168,22 @@ app = FastAPI(
     root_path=config.root_path,
 )
 
+# Rate limiter (in-memory storage, suitable for single-instance deployments)
+limiter = Limiter(key_func=get_remote_address)
+app.state.limiter = limiter
+
+
+@app.exception_handler(RateLimitExceeded)
+async def rate_limit_handler(request: Request, exc: RateLimitExceeded):
+    """Return 429 with Retry-After header when rate limit is exceeded."""
+    retry_after = getattr(exc, "retry_after", 60)
+    return JSONResponse(
+        status_code=429,
+        content={"detail": "Rate limit exceeded. Please try again later."},
+        headers={"Retry-After": str(retry_after)},
+    )
+
+
 # Add CORS middleware for React frontend
 app.add_middleware(
     CORSMiddleware,
@@ -178,7 +198,8 @@ app.add_middleware(
 
 
 @app.post("/auth/register", response_model=UserResponse, tags=["Auth"])
-async def register(request: RegisterRequest):
+@limiter.limit("5/minute")
+async def register(request: Request, body: RegisterRequest):
     """Register a new user.
 
     The first registered user automatically becomes an admin.
@@ -190,8 +211,8 @@ async def register(request: RegisterRequest):
     role = "admin" if user_count == 0 else "user"
 
     user = db.create_user(
-        email=request.email,
-        password=request.password,
+        email=body.email,
+        password=body.password,
         role=role,
     )
 
@@ -210,11 +231,12 @@ async def register(request: RegisterRequest):
 
 
 @app.post("/auth/login", response_model=TokenResponse, tags=["Auth"])
-async def login(request: LoginRequest):
+@limiter.limit("10/minute")
+async def login(request: Request, body: LoginRequest):
     """Authenticate user and return JWT tokens."""
     db = get_db_client()
 
-    user = db.authenticate_user(request.email, request.password)
+    user = db.authenticate_user(body.email, body.password)
 
     if not user:
         raise HTTPException(