Allow background task result TTL to be configured via environment variable

[carterthayer]

Enhancement

Problem

FastMCP exposes several Docket worker settings via FASTMCP_DOCKET_* environment variables, but the result TTL is not among them.
Operators running long-duration background tasks have no way to extend how long results are retained in Redis without modifying installed package code.

Use case

A server runs multi-hour background tasks. The default 15-minute TTL means results may expire before clients retrieve them. There's currently no supported way to increase retention to match the actual task duration.

Expected behavior

Something like FASTMCP_DOCKET_EXECUTION_TTL that controls how long completed task results are kept, consistent with how other Docket settings are already configurable.

[carterthayer]

Additionally, it would be useful to decouple the task metadata TTL from the result TTL.

I'd like to keep the result around for multiple days, but the task metadata (in particular snapshotted user tokens) should live only the length of the task.