Add per-IP rate limiting to prevent finger-daemon abuse

The app sits behind nginx, which caches 200s for 30s — so repeated lookups
of the same user are cheap. What bypasses the cache is enumeration of distinct
usernames: each is a unique cache key -> miss -> a fresh finger call to the
mammut daemon, all attributed to admin's single IP (so the daemon cannot ban
the real source). The app only ever receives cache misses, so a per-IP limit
here throttles exactly that uncached path without touching the cached hot path.

- Flask-Limiter keyed per client IP: 30/min on the finger lookup endpoints,
  10/min on /api/upload (auth brute-force), 120/min global default. Index and
  the container healthcheck are exempt. All limits env-tunable (RATELIMIT_*).
- ProxyFix(x_for=1): trust nginx's X-Forwarded-For so the real client IP is
  used for keying and logging. Without it the app only saw the Docker bridge
  gateway (172.20.0.1) and every client shared one bucket.
- 429 handler (JSON for /api, HTML 429.html otherwise) and WARNING logging of
  failed/invalid lookups and limit hits, so enumeration is observable.

templates/429.html

@@ -0,0 +1,26 @@
+{% extends "base.html" %}
+
+{% block content %}
+<div class="row">
+    <div class="col-lg-6 mx-auto text-center">
+        <div class="card">
+            <div class="card-body">
+                <h1 class="display-1 text-muted">429</h1>
+                <h2 class="mb-3">Too Many Requests</h2>
+                <p class="lead">
+                    You've sent too many requests in a short period. Please slow
+                    down and try again in a moment.
+                </p>
+                {% if detail %}
+                <p class="text-muted"><small>Limit: {{ detail }}</small></p>
+                {% endif %}
+                <hr class="my-4">
+                <div class="d-grid gap-2 d-md-block">
+                    <a href="{{ url_for('index') }}" class="btn btn-primary">Go Home</a>
+                    <a href="javascript:history.back()" class="btn btn-outline-secondary">Go Back</a>
+                </div>
+            </div>
+        </div>
+    </div>
+</div>
+{% endblock %}