P7: Implement Redis RPM, TPM, and concurrency primitives

[wauputr4]

Goal

Implement Redis-backed counters and leases for requests per minute, tokens per minute, and concurrency limits.

Acceptance Criteria

• Per-key RPM counter works with TTL.
• Per-key TPM counter works with TTL.
• Per-key, per-user, and per-provider concurrency leases are supported.
• Crashed requests do not permanently hold leases.