Epic: AI Cost and Budget Controls

[shaneutt]

Track and enforce cost budgets for LLM inference traffic based on token consumption.

Scope:

• Input token estimation before forwarding (pre-request cost check)
• Per-client/per-route token budgets with configurable windows
• Cost attribution: map token usage to user, session, model, and endpoint
• Budget exceeded behavior: reject with 429 or queue
• Cost data exposed to access logging and metrics

[shaneutt]

note: this may need merging in with other epics, as there's multiple overlaps with cost and budget and other work we're doing.