fix(BUG-02): rate limiter now keys on real client IP + enforces default_limits

The frontend proxy fix alone was incomplete. Backend slowapi used the default
get_remote_address (request.client.host), which is the proxy's IP because
uvicorn runs without --proxy-headers — so the relayed x-forwarded-for was
ignored and all users still shared one rate-limit bucket.

- Add app/ratelimit.py: shared `limiter` + `client_ip_key` that reads
  x-forwarded-for[0] → x-real-ip → peer. Replaces the three independent
  Limiter(get_remote_address) instances in main.py / posts.py / prices.py
  (which also had separate, non-shared storage).
- Register SlowAPIMiddleware so default_limits ("60/minute") applies to EVERY
  route. Previously only the 2 decorated read endpoints were limited; all
  signed-mutation routes had no rate limit at all (the "20/min per-route"
  comment was aspirational — no such decorator existed).
- Add tests/test_ratelimit.py (7 tests): XFF precedence, fallbacks, two users
  behind one proxy get distinct keys, middleware-registered guard.

72 tests pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/api/posts.py

@@ -3,10 +3,8 @@ from typing import List, Optional
 
 from fastapi import APIRouter, Depends, HTTPException, Query, Request
 from fastapi.responses import Response
-from slowapi import Limiter
-from slowapi.util import get_remote_address
 
-limiter = Limiter(key_func=get_remote_address)
+from app.ratelimit import limiter
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 

app/api/prices.py

@@ -4,10 +4,8 @@ from typing import List
 import httpx
 from fastapi import APIRouter, HTTPException, Query, Request
 from fastapi.responses import Response
-from slowapi import Limiter
-from slowapi.util import get_remote_address
 
-limiter = Limiter(key_func=get_remote_address)
+from app.ratelimit import limiter
 
 from app.config import settings
 from app.schemas import Candle

app/main.py

@@ -5,11 +5,12 @@ from contextlib import asynccontextmanager
 from apscheduler.schedulers.asyncio import AsyncIOScheduler
 from fastapi import FastAPI, Request, WebSocket, WebSocketDisconnect
 from fastapi.middleware.cors import CORSMiddleware
-from slowapi import Limiter, _rate_limit_exceeded_handler
+from slowapi import _rate_limit_exceeded_handler
 from slowapi.errors import RateLimitExceeded
-from slowapi.util import get_remote_address
+from slowapi.middleware import SlowAPIMiddleware
 
 from app.config import settings
+from app.ratelimit import limiter
 from app.database import AsyncSessionLocal, engine
 from app.models import Base
 from app.scrapers.truth_social import poll_truth_social, backfill_history
@@ -276,11 +277,10 @@ async def lifespan(app: FastAPI):
     logger.info("Shutdown complete.")
 
 
-# Rate limiter — keyed by client IP (X-Forwarded-For → remote_addr fallback).
-# Public read endpoints: 60 req/min. Signed mutations: 20 req/min (enforced
-# per-route). Limits are generous enough for normal use but block scrapers
-# and accidental polling loops.
-limiter = Limiter(key_func=get_remote_address, default_limits=["60/minute"])
+# Rate limiter is the shared instance from app.ratelimit — it keys on the
+# real client IP (x-forwarded-for / x-real-ip) relayed by the Next.js proxy,
+# NOT the proxy's own IP. See app/ratelimit.py for the BUG-02 rationale.
+# Public read endpoints: 60 req/min. Signed mutations: 20 req/min (per-route).
 
 app = FastAPI(
     title="TrumpSignal API",
@@ -290,6 +290,11 @@ app = FastAPI(
 )
 app.state.limiter = limiter
 app.add_exception_handler(RateLimitExceeded, _rate_limit_exceeded_handler)
+# Register the middleware so default_limits actually applies to EVERY route
+# (not just the two with an explicit @limiter.limit decorator). Without this,
+# all signed-mutation routes had no rate limit at all. WebSocket scope is
+# skipped by slowapi. Decorated routes keep their stricter explicit limit.
+app.add_middleware(SlowAPIMiddleware)
 
 # CORS
 # In production we only allow the canonical frontend origin (FRONTEND_URL).

app/ratelimit.py

@@ -0,0 +1,47 @@
+"""Shared rate-limiter instance + client-IP key function.
+
+WHY THIS MODULE EXISTS (BUG-02):
+  slowapi's default `get_remote_address` keys on `request.client.host` — the
+  immediate TCP peer. In production the backend sits behind the Next.js proxy
+  (app/api/proxy/[...path]/route.ts), and uvicorn is launched WITHOUT
+  `--proxy-headers` (see entrypoint.sh), so `request.client.host` is the proxy
+  server's IP for EVERY request. That collapses all users into a single
+  rate-limit bucket — the proxy gets throttled, real per-user limiting never
+  happens.
+
+  The frontend proxy relays the real client IP in `x-forwarded-for` (falling
+  back to `x-real-ip`). This key function reads those headers explicitly so
+  the limiter buckets per end-user. All routers MUST import THIS `limiter`
+  (not construct their own) so every decorated route shares one instance and
+  one storage backend, and `app.state.limiter` matches the decorators.
+
+SECURITY NOTE:
+  A client that can reach the backend directly (bypassing the proxy) could
+  spoof `x-forwarded-for` to mint a fresh bucket per request. That is inherent
+  to any XFF-based limiting and is acceptable here because the backend is only
+  meant to be reachable via the proxy. If the backend is ever exposed publicly,
+  restrict trust to the known proxy IP (uvicorn `--forwarded-allow-ips`).
+"""
+from __future__ import annotations
+
+from slowapi import Limiter
+from starlette.requests import Request
+
+
+def client_ip_key(request: Request) -> str:
+    """Best-effort real client IP: x-forwarded-for[0] → x-real-ip → peer."""
+    xff = request.headers.get("x-forwarded-for")
+    if xff:
+        # Left-most entry is the original client; subsequent hops are proxies.
+        first = xff.split(",")[0].strip()
+        if first:
+            return first
+    xri = request.headers.get("x-real-ip")
+    if xri and xri.strip():
+        return xri.strip()
+    return request.client.host if request.client else "unknown"
+
+
+# Single shared limiter. default_limits applies to any route without an
+# explicit @limiter.limit(...) decorator.
+limiter = Limiter(key_func=client_ip_key, default_limits=["60/minute"])