[Docs] Fix stale REDIS_URL docs, it is already used for rate limiting

[collinsezedike]

Summary

apps/docs/operations/environment-variables.md lists REDIS_URL under a "Future variables" section as currently not used. It is already used: apps/api-local/src/index.ts reads process.env.REDIS_URL and wires it into the @fastify/rate-limit plugin as its store.

Motivation

Anyone setting up a new environment and reading this doc would conclude REDIS_URL is optional/unused and skip configuring it, when in practice the API's rate limiting depends on it.

Proposed Solution

Move the REDIS_URL row out of "Future variables" into the main environment variable table, with a description matching its actual role (rate-limit store for apps/api-local).

Scope

Field	Value
Area	Docs, API
Protocol affected	None
Network	N/A
Breaking change?	No

Alternatives Considered

None, this is a straightforward factual correction.

Acceptance Criteria

• REDIS_URL appears in the main environment variable table, not the future/unused section
• Description accurately reflects its use as the rate-limit store

Additional Context

Verified against apps/api-local/src/index.ts (reads process.env.REDIS_URL, wires into @fastify/rate-limit).

[Caneryy]

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