fix(core): remove unwrap/expect in non-test code

- cache/lru.rs: replace lock().unwrap() with if let Ok guard,
  consistent with other lock acquisitions in the same file
- service/repo/core.rs: replace try_into().unwrap() with
  copy_from_slice which is infallible for fixed-size slices
- service/auth/rsa.rs: replace 3 expect() calls with map_err()
  for ChaCha20Poly1305 key init and session key retrieval
- config/mod.rs: replace GLOBAL_CONFIG.get().expect() with
  unwrap_or_else fallback to empty config

cache/lru.rs

@@ -174,22 +174,22 @@ impl<K: Eq + Hash + Clone, V: Clone> LruTtlCache<K, V> {
             return;
         }
 
-        let mut lru = self.lru.lock().unwrap();
+        if let Ok(mut lru) = self.lru.lock() {
+            if lru.len() >= self.capacity
+                && let Some(evicted_key) = lru.pop_back()
+            {
+                self.map.remove(&evicted_key);
+            }
 
-        if lru.len() >= self.capacity
-            && let Some(evicted_key) = lru.pop_back()
-        {
-            self.map.remove(&evicted_key);
+            self.map.insert(
+                key.clone(),
+                CacheEntry {
+                    value,
+                    expires_at: now + ttl,
+                },
+            );
+            lru.push_front(key);
         }
-
-        self.map.insert(
-            key.clone(),
-            CacheEntry {
-                value,
-                expires_at: now + ttl,
-            },
-        );
-        lru.push_front(key);
     }
 
     pub fn remove(&self, key: &K) -> Option<V> {