fix(#338): Log ERROR on rate limiter fallback and track degraded mode

- Log at ERROR level when falling back to in-memory storage
- Track and expose degraded mode status for health checks
- Add isUsingFallback() method to check fallback state
- Add getHealthStatus() method for health check endpoints
- Add comprehensive tests for fallback behavior and health status

Refs #338

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/api/src/common/throttler/throttler-storage.service.ts

@@ -53,8 +53,11 @@ export class ThrottlerValkeyStorageService implements ThrottlerStorage, OnModule
       this.logger.log("Valkey connected successfully for rate limiting");
     } catch (error) {
       const errorMessage = error instanceof Error ? error.message : String(error);
-      this.logger.warn(`Failed to connect to Valkey for rate limiting: ${errorMessage}`);
-      this.logger.warn("Falling back to in-memory rate limiting storage");
+      this.logger.error(`Failed to connect to Valkey for rate limiting: ${errorMessage}`);
+      this.logger.error(
+        "DEGRADED MODE: Falling back to in-memory rate limiting storage. " +
+          "Rate limits will not be shared across API instances."
+      );
       this.useRedis = false;
       this.client = undefined;
     }
@@ -168,6 +171,46 @@ export class ThrottlerValkeyStorageService implements ThrottlerStorage, OnModule
     return `${this.THROTTLER_PREFIX}${key}`;
   }
 
+  /**
+   * Check if the service is using fallback in-memory storage
+   *
+   * This indicates a degraded state where rate limits are not shared
+   * across API instances. Use this for health checks.
+   *
+   * @returns true if using in-memory fallback, false if using Redis
+   */
+  isUsingFallback(): boolean {
+    return !this.useRedis;
+  }
+
+  /**
+   * Get rate limiter health status for health check endpoints
+   *
+   * @returns Health status object with storage mode and details
+   */
+  getHealthStatus(): {
+    healthy: boolean;
+    mode: "redis" | "memory";
+    degraded: boolean;
+    message: string;
+  } {
+    if (this.useRedis) {
+      return {
+        healthy: true,
+        mode: "redis",
+        degraded: false,
+        message: "Rate limiter using Redis storage (distributed mode)",
+      };
+    }
+    return {
+      healthy: true, // Service is functional, but degraded
+      mode: "memory",
+      degraded: true,
+      message:
+        "Rate limiter using in-memory fallback (degraded mode - limits not shared across instances)",
+    };
+  }
+
   /**
    * Clean up on module destroy
    */