feat: user-defined LLM committees

[duncanmcqueen]

Summary

Adds user-defined LLM committees: a committee runs its members in parallel and a chair synthesizes the result, configurable per user under Settings > Models and persisted on the profile (user_profiles.model_committees).

Built on the model-registry foundation (#339).

What's added

Backend

• lib/llm/committee.ts — parallel member execution + chair synthesis, with cycle and self-chair guards.
• lib/userCommittees.ts — validation/normalization of user committee configs (max 8 committees, max 8 members).
• lib/llm/registry.ts — getCommitteeModel and friends accept a backward-compatible optional additionalCommittees array so user committees resolve like configured ones.
• lib/llm/{models,index}.ts — committee-aware model resolution, availability checks, and chat/complete dispatch.
• lib/chat/streaming.ts — loads the user's committees and resolves a usable model through them.
• routes/user.ts — model_committees on the profile (select/serialize/PATCH) plus GET /user/models; profile reads tolerate pre-migration databases.
• routes/tabular.ts — committee-aware model-availability messages.
• migrations/20260813_02_user_model_committees.sql + schema.sql.

Frontend

• Settings > Models — create/edit/delete committees (members + chair), shown in both model pickers.
• ModelToggle — a "Committee" group listing the user's committees, with reset-to-default when a committee is removed.
• mikeApi / UserProfileContext — committee types, profile state, and save.

Validation

• backend: tsc --noEmit clean; vitest run 655 passed, 24 skipped (+14 committee tests).
• frontend: tsc --noEmit clean; vitest run 292 passed + 4 pre-existing Blob.text() failures in untouched mikeApi.test.ts.

This PR is stacked on #339 (feat/model-registry-foundation); retarget to main once that merges.

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