Review: Use structured output for tag enrichment

Change summary

Replaces free-form provider.complete("tag-extraction", …) with a dedicated provider.extractTags() method that uses the xAI/Grok Responses API json_schema structured output mode. The contract layer gains a Zod schema (tagExtractionResultSchema) and a mirrored JSON Schema (tagExtractionJsonSchema) to validate the LLM response at both the API constraint level and at parse time.

Files touched: 3 — contracts/src/index.ts (new schemas), ai/src/index.ts (new method), web/trigger/tasks.ts (call-site simplified).

Validation confidence: High

• All tests pass (7/7 packages)
• Type-check clean across the monorepo (including Next.js build)
• Zod parse after JSON.parse provides a runtime safety net even if the model drifts
• .max(8) on the Zod array matches the system prompt instruction ("up to 8")

Remaining risks

1. Dual-schema drift — The Zod schema and the hand-written JSON Schema are defined independently. If one is updated and the other isn't, the API constraint and runtime validation will diverge. Consider generating one from the other (e.g. zod-to-json-schema).
2. No maxItems in the JSON Schema — The Zod side enforces .max(8), but the JSON Schema sent to the model doesn't include "maxItems": 8, so the model could return >8 tags that then fail Zod parsing at runtime.
3. JSON.parse on missing output_text — Falls back to "{}" which parses to {}, then Zod would accept { tags: undefined } … except .array() won't coerce undefined. This will throw a ZodError rather than returning { tags: [] } like the disabled-provider path does. A graceful fallback or explicit check might be safer.
4. No timeout / retry — The fetch call has no AbortSignal timeout. A hung upstream will block the Trigger task indefinitely.
5. Slug format not validated — The slug field is z.string() with no regex constraint for URL-safety, relying entirely on the model to obey the system prompt.

None of these are blockers — the change is a clear improvement over the unstructured path. Items 2 and 3 are the most actionable quick fixes.