WP-O2b: origin derivation (no live tag touched)

[byteclimber]

Part of the origin-dimension implementation plan: adobe/serenity-docs#46 (branch feat/origin_source).

Touches no tag tree, no live data. May share WP-O2a's PR or follow it.

• handlers/prompts.js:384: generalize makeTypeInjector to strip+inject type and origin, with create/update asymmetry — create injects derived value, update re-injects the stored origin and never re-derives. Strip by resolved tag id beneath the root, never by name.
• prompts-storage.js:792 upsertPrompts: write the origin derived from the request's principal — service principal: asserted body value, validated; user principal: always human, body ignored.
• prompts-storage.js:1006-1007 updatePromptById: stop patching origin from the body.
• prompts-storage.js:408 mapRowToPrompt: return row.origin directly, drop the || 'human' fallback (zero NULLs in prod, spec §2.3 — the fallback would now silently mislabel).
• src/controllers/brands.js: createPromptsByBrand derives origin from principal. Origin query filter/sort key stay.
• docs/openapi/schemas.yaml / prompts-v2-api.yaml: origin stays on V2PromptInput/V2Prompt, re-documented as service-principal-only; PATCH body no longer honours it.
• No change needed in llmo-data-retrieval-service — its V2Prompt model pins origin: Literal["ai"] and POSTs as a service principal; that stays correct.

Exit: unit + it-postgres integration green. Spec gates 5,6,7,8 land here.

Blocked on the tag-vs-metadata product decision.

Jira: SITES-48002