style(backend): rustfmt the whole tree; gate cargo fmt --check in CI

The backend had never been run through rustfmt. Doing it in one mechanical pass (134 files)
so no future functional diff is buried under formatting churn, then gating `cargo fmt
--check` in checks.yml so it stays clean.

Formatting only — no logic, SQL, or behaviour changed. Verified after the reformat:
cargo test 56 passed, clippy --all-targets -D warnings clean, cargo fmt --check clean.

This is the deferred cleanup noted when CI's Format step was first left out.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/services/config.rs

@@ -81,17 +81,18 @@ impl ConfigCache {
         }
 
         // Cache miss or stale — reload the entire table in one query.
-        let rows: Vec<(String, String)> =
-            match sqlx::query_as::<_, (String, String)>("SELECT key, value FROM config")
-                .fetch_all(&self.pool)
-                .await
-            {
-                Ok(rows) => rows,
-                Err(e) => {
-                    tracing::warn!(error = ?e, "config reload failed; using defaults for this read");
-                    return None;
-                }
-            };
+        let rows: Vec<(String, String)> = match sqlx::query_as::<_, (String, String)>(
+            "SELECT key, value FROM config",
+        )
+        .fetch_all(&self.pool)
+        .await
+        {
+            Ok(rows) => rows,
+            Err(e) => {
+                tracing::warn!(error = ?e, "config reload failed; using defaults for this read");
+                return None;
+            }
+        };
 
         let values: HashMap<String, String> = rows.into_iter().collect();
         let result = values.get(key).cloned();
@@ -104,26 +105,43 @@ impl ConfigCache {
 }
 
 pub async fn get_str(cache: &ConfigCache, key: &str, default: &str) -> String {
-    cache.get_raw(key).await.unwrap_or_else(|| default.to_string())
+    cache
+        .get_raw(key)
+        .await
+        .unwrap_or_else(|| default.to_string())
 }
 
 pub async fn get_i64(cache: &ConfigCache, key: &str, default: i64) -> i64 {
-    cache.get_raw(key).await.and_then(|v| v.parse().ok()).unwrap_or(default)
+    cache
+        .get_raw(key)
+        .await
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(default)
 }
 
 pub async fn get_usize(cache: &ConfigCache, key: &str, default: usize) -> usize {
-    cache.get_raw(key).await.and_then(|v| v.parse().ok()).unwrap_or(default)
+    cache
+        .get_raw(key)
+        .await
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(default)
 }
 
 pub async fn get_f64(cache: &ConfigCache, key: &str, default: f64) -> f64 {
-    cache.get_raw(key).await.and_then(|v| v.parse().ok()).unwrap_or(default)
+    cache
+        .get_raw(key)
+        .await
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(default)
 }
 
 /// Parses common truthy spellings used by both the migration seeds and the admin form.
 /// Accepts `true/false`, `1/0`, `yes/no`, `on/off` — case-insensitive. Anything else
 /// returns `default`.
 pub async fn get_bool(cache: &ConfigCache, key: &str, default: bool) -> bool {
-    let Some(raw) = cache.get_raw(key).await else { return default };
+    let Some(raw) = cache.get_raw(key).await else {
+        return default;
+    };
     match raw.trim().to_ascii_lowercase().as_str() {
         "true" | "1" | "yes" | "on" => true,
         "false" | "0" | "no" | "off" => false,