feat: Implement basic caching layer with cache-aside pattern for /search results

- Implement cache-aside pattern via SearchCacheService in RouteSearch handler
- Add TTL policies: 3 hours near-term, 7 days far-term
- Write TestCacheAsideSearch and variants for TTL verification
- Extend CacheAside to fully implement Cache interface (Get, Set, Exists, Delete, Increment, Decrement)

internal/cache/store.go

@@ -228,3 +228,47 @@ func (c *CacheAside) InvalidateStation(ctx context.Context, key *CacheKey) error
 func (c *CacheAside) InvalidateSearch(ctx context.Context, key *CacheKey) error {
 	return c.store.Delete(ctx, key)
 }
+
+// Delete removes a key from cache.
+func (c *CacheAside) Delete(ctx context.Context, key *CacheKey) error {
+	return c.store.Delete(ctx, key)
+}
+
+// Get retrieves a value from cache by key.
+func (c *CacheAside) Get(ctx context.Context, key *CacheKey) ([]byte, error) {
+	val, err := c.store.Get(ctx, key)
+	if errors.Is(err, redis.Nil) {
+		return nil, nil // cache miss
+	}
+	if err != nil {
+		return nil, fmt.Errorf("cache get: %w", err)
+	}
+	return val, nil
+}
+
+// Set stores a value in cache with an expiry TTL.
+func (c *CacheAside) Set(ctx context.Context, key *CacheKey, value []byte, ttl time.Duration) error {
+	return c.store.Set(ctx, key, value, ttl)
+}
+
+// Exists checks if a key exists in cache.
+func (c *CacheAside) Exists(ctx context.Context, key *CacheKey) (bool, error) {
+	_, err := c.store.Exists(ctx, key)
+	if errors.Is(err, redis.Nil) {
+		return false, nil
+	}
+	if err != nil {
+		return false, fmt.Errorf("cache exists: %w", err)
+	}
+	return true, nil
+}
+
+// Increment increments a counter key.
+func (c *CacheAside) Increment(ctx context.Context, key *CacheKey) (int64, error) {
+	return c.store.Increment(ctx, key)
+}
+
+// Decrement decrements a counter key.
+func (c *CacheAside) Decrement(ctx context.Context, key *CacheKey) (int64, error) {
+	return c.store.Decrement(ctx, key)
+}