feat: add System Prompts library for Expand Prompt button

[Pfannkuchensack]

Summary

• Add system_prompts SQLite table (migration 32) seeded with 7 curated default prompts: an InvokeAI Default (mirrors the hardcoded fallback in text_llm_pipeline.DEFAULT_SYSTEM_PROMPT) plus 6 prompts adapted from FLUX.2, HunyuanImage 3.0, Qwen-Image (edit + 2512), Z-Image and HiDream
• Add CRUD service layer + REST router at /api/v1/system_prompts, with per-user ownership and optional sharing (modeled on the workflow library): user_id + is_public columns, CurrentUserOrDefault dependency on every endpoint, owner-or-admin checks for PATCH/DELETE, and own+public scoping for LIST in multi-user mode
• Add RTK Query endpoints, management modal (list/create/edit/delete) and a system-prompt picker in the Expand Prompt popover
• Persist last picked system prompt + text-LLM model via Redux (expandPrompt slice)
• Frontend respects ownership: useCanEditSystemPrompt hook hides edit/delete on prompts the user does not own, list items show System / Shared badges, and the form exposes a Share with everyone toggle when multi-user is enabled
• Adds a sibling node to TextLLMInvocation that takes a SystemPromptField (a
  DB-backed preset reference) instead of a free-text system prompt.

Related Issues / Discussions

None.

QA Instructions

Backend — single-user (default)

1. Start a fresh dev server: uv run --extra cuda invokeai-web — migration 32 runs, creates the system_prompts table (with user_id + is_public columns) and seeds 7 default prompts as user_id='system', is_public=TRUE.
2. Verify via REST:
   • GET /api/v1/system_prompts/ → 7 records
   • POST /api/v1/system_prompts/ with {"name":"Test","content":"foo"} → 200; record echoed back with is_public=true (single-user default)
   • PATCH /api/v1/system_prompts/i/{id} with {"name":"Test2"} → record updated
   • DELETE /api/v1/system_prompts/i/{id} → record gone; subsequent GET returns 404
3. Restart the app — deleted defaults stay deleted (fixed UUIDs + INSERT OR IGNORE).

Backend — multi-user
4. Run with multi-user enabled and create two non-admin users (alice, bob) plus an admin.
5. As alice: POST a private prompt → it is owned by alice, is_public=false.
6. As bob: GET /api/v1/system_prompts/ → sees the 7 system defaults plus only public prompts; does not see alice's private prompt.
7. As bob: PATCH or DELETE of alice's prompt id → 403 (or 404 if he does not even know the id, depending on path).
8. As alice: PATCH … {"is_public": true} → flips her prompt to shared; bob's next GET now includes it.
9. As admin: GET returns all prompts (including alice/bob's privates), and PATCH/DELETE on any prompt succeeds.

Frontend
10. Install at least one Text-LLM model (e.g. via the Model Manager starter list).
11. In the Generate tab, click the Expand Prompt button (sparkle icon) over a non-empty prompt.
12. The popover now shows a System Prompt combobox above the model picker — pick one of the seeded defaults, pick a model, click Expand. The positive prompt is replaced; Ctrl+Z restores the original.
13. Click the pencil icon next to the system-prompt combobox → management modal opens.
- Single-user: Create / Edit / Delete buttons appear on every row.
- Multi-user (non-admin): Edit / Delete only appear on prompts you own; system/shared prompts show a badge instead.
- Multi-user: opening the form for an owned prompt shows the Share with everyone checkbox.
14. Reload the browser → the previously picked system prompt and model are still selected (Redux persistence).
15. Lint + test suite: pnpm lint && pnpm test:no-watch from invokeai/frontend/web/ (1 pre-existing flaky timer test in navigation-api.test.ts is unrelated).

Merge Plan

This PR adds a new SQLite migration (32) and a new persisted Redux slice. Notes for reviewers / mergers:

• Migration 32 creates the system_prompts table with user_id + is_public already in the schema and seeds 7 default rows owned by system (public). For dev databases that already ran an earlier revision of this migration without the multi-user columns, the migration also runs PRAGMA table_info and ALTER TABLE … ADD COLUMN to backfill the columns — safe and idempotent.
• The migration only touches its own table; no impact on workflows, style presets, etc.
• Redux slice migration: the new expandPrompt slice has a migrate step (sets _version: 1 if missing) — an old persisted state without this slice simply starts from initial state, no breaking change for users.
• No coordination with other PRs needed; standard merge.

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)