Feature: Configurable AI model selection

[Buooy]

Summary

AI model choices are currently hard-coded in the Worker code. We should centralise model selection so the email agent, auto-drafting flow, prompt-injection scanner, and draft verifier can each use a default model while also supporting deploy-time overrides.

Proposed Changes

• Add a centralized AI model config module with defaults for:
  • Email agent chat
  • Auto-draft generation
  • Prompt-injection scanning
  • Draft verification
• Support Worker env var overrides for each model.
• Replace hard-coded model strings in agent and AI helper call sites.
• Add the corresponding model variables to wrangler.jsonc.

Acceptance Criteria

• Model defaults are defined in one place.
• Existing behavior remains unchanged when no overrides are provided.
• Each model can be overridden via Worker vars.
• Typecheck passes.