Add RedisBackendCondition for Spring conditional bean wiring

Portable Spring Condition that matches when rqueue.backend resolves
to REDIS (default when the property is absent or unparseable).
Companion to the existing rqueue-spring NatsBackendCondition; lives in
rqueue-core so both Boot and non-Boot modules can reuse it without
crossing module boundaries.

Per-bean gating (applying @conditional(RedisBackendCondition.class) to
every Redis-shaped bean factory) is staged separately; this commit
adds only the condition class itself.

Assisted-By: Claude Code

Author: sonus21

rqueue-core/src/main/java/com/github/sonus21/rqueue/config/RedisBackendCondition.java

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2024-2026 Sonu Kumar
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * You may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ */
+
+package com.github.sonus21.rqueue.config;
+
+import org.springframework.context.annotation.Condition;
+import org.springframework.context.annotation.ConditionContext;
+import org.springframework.core.type.AnnotatedTypeMetadata;
+
+/**
+ * Spring {@link Condition} that matches when the active rqueue backend is {@link Backend#REDIS}.
+ * Reads the {@code rqueue.backend} property; absent or unparseable values default to REDIS so the
+ * existing behavior is preserved.
+ */
+public class RedisBackendCondition implements Condition {
+  @Override
+  public boolean matches(ConditionContext ctx, AnnotatedTypeMetadata md) {
+    return resolveBackend(ctx) == Backend.REDIS;
+  }
+
+  static Backend resolveBackend(ConditionContext ctx) {
+    String raw = ctx.getEnvironment().getProperty("rqueue.backend");
+    if (raw == null || raw.isBlank()) {
+      return Backend.REDIS;
+    }
+    try {
+      return Backend.valueOf(raw.trim().toUpperCase());
+    } catch (IllegalArgumentException ex) {
+      return Backend.REDIS;
+    }
+  }
+}