feat: implement rate limiting across all API endpoints

Add sliding-window in-memory RateLimiter service (Arc<Mutex<HashMap>>)
with per-IP and per-user-id limits on all public endpoint classes:
- POST /api/v1/join: 5/min per IP
- GET /api/v1/feed: configurable per IP (feed_rate_per_min, default 60)
- POST /api/v1/upload: configurable per user (upload_rate_per_hour, default 10)
- GET /api/v1/export/zip|html: configurable per IP (export_rate_per_day, default 3)
Limits are hot-reloadable via the config table. All 429 responses use
German error messages. Client IP is read from X-Forwarded-For (Caddy).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/services/rate_limiter.rs

@@ -0,0 +1,46 @@
+use std::collections::HashMap;
+use std::sync::{Arc, Mutex};
+use std::time::{Duration, Instant};
+
+/// Thread-safe sliding-window rate limiter backed by an in-memory HashMap.
+/// Each key (e.g. `"join:{ip}"` or `"upload:{user_id}"`) tracks timestamps
+/// of recent requests and rejects new ones once the window is full.
+#[derive(Clone)]
+pub struct RateLimiter {
+    windows: Arc<Mutex<HashMap<String, Vec<Instant>>>>,
+}
+
+impl RateLimiter {
+    pub fn new() -> Self {
+        Self {
+            windows: Arc::new(Mutex::new(HashMap::new())),
+        }
+    }
+
+    /// Returns `true` if the request is allowed, `false` if rate-limited.
+    pub fn check(&self, key: impl Into<String>, max: usize, window: Duration) -> bool {
+        let now = Instant::now();
+        let key = key.into();
+        let mut map = self.windows.lock().unwrap();
+        let timestamps = map.entry(key).or_default();
+        // Drop entries outside the window
+        timestamps.retain(|&t| now.duration_since(t) < window);
+        if timestamps.len() < max {
+            timestamps.push(now);
+            true
+        } else {
+            false
+        }
+    }
+}
+
+/// Extract the client IP from X-Forwarded-For (Caddy sets this) or fall back
+/// to a provided socket address string.
+pub fn client_ip(headers: &axum::http::HeaderMap, fallback: &str) -> String {
+    headers
+        .get("x-forwarded-for")
+        .and_then(|v| v.to_str().ok())
+        .and_then(|s| s.split(',').next())
+        .map(|s| s.trim().to_owned())
+        .unwrap_or_else(|| fallback.to_owned())
+}