Add example format to REDIS_URL environment variable documentation

[Copilot]

Addresses feedback from #1982 to include example format for the REDIS_URL environment variable.

Changes

• Updated REDIS_URL documentation in environment variables table to include example format (example: redis://host:port), consistent with other DSN-style variables like MESSENGER_TRANSPORT_DSN

-| `REDIS_URL` | (empty) | Redis connection URL for caching and sessions |
+| `REDIS_URL` | (empty) | Redis connection URL for caching and sessions (example: `redis://host:port`) |

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