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/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