fix(platform): clarify builder cost display for OpenRouter and free providers (#13091)

### Why / What / How

**Why:** The AI Text Generator (and every other LLM block) rendered the
raw `cost_amount=150` for OpenRouter-routed models as if it were per-run
credits → the node card showed `~$1.50/run` for every OR model
regardless of which one the user picked. Backend billing was always
correct (post-flight settles against `x-total-cost`), but the pre-flight
display made cheap models like Qwen 3 look 10× more expensive than
Claude Opus, and Ollama / Z.ai free models had no cost label at all.
Surfaced by Craig in Discord thread
[#1503736802299875441](https://discord.com/channels/1126875755960336515/1503736802299875441/1503736859782811710);
follow-up feedback from Krzysztof (UX is unclear) and Toran (users
shouldn't have to learn the difference between "billed by tokens" vs
"billed by USD").

**What:** Replace the misleading cost amount with the provider's
*published* per-1M-token USD rate, rendered the same way for every LLM
regardless of internal billing path. Show "Free" for Ollama. Show
"Pay-as-you-go" only when we genuinely don't have a published rate.

**How:**
- `BlockCost` now carries optional `input_usd_per_1m`,
`output_usd_per_1m`, `cache_read_usd_per_1m`,
`cache_creation_usd_per_1m` (USD/1M, derived from the existing
`TOKEN_COST` credit/1M rates by dividing by 150 — the cr-per-$
multiplier that already encodes our 1.5× margin). Display-only; the
billing path (`compute_token_credits` / `_coerce_usd × cost_amount`) is
unchanged.
- All TOKENS-typed LLM_COST entries (Anthropic / OpenAI / Groq / AIML /
Llama-API / v0) get rates wired through a new `_tokens_llm_cost()`
helper.
- OpenRouter COST_USD entries *also* get rates wired when the model has
a `TOKEN_COST` row — so OR-billed models (Mistral, Kimi, Cohere, Grok,
Gemini, DeepSeek, Perplexity) display the same `$X.XX in / $Y.YY out per
1M tokens` as direct-billed providers.
- New explicit Ollama loop emits `cost_type=RUN, cost_amount=0` for each
Ollama model so the node card renders "Free" instead of an empty label.
- Added missing `CLAUDE_4_7_OPUS` row to `TOKEN_COST` (was falling back
to flat $0.14/run).
- Frontend `NodeCost.tsx` rewritten to:
- Prefer the rate-pair display (`$X in / $Y out per 1M tokens`) whenever
rates are present, regardless of `cost_type`.
- Tooltip lists cached/cache-write rates when provider publishes them
(Anthropic).
  - "Pay-as-you-go" for COST_USD without rates (no fake number).
  - "Free" for RUN with `cost_amount=0`.

### Changes 🏗️

- `backend/blocks/_base.py`: Add four optional USD/1M rate fields to
`BlockCost`.
- `backend/data/block_cost_config.py`: New `_token_usd_rates()` and
`_tokens_llm_cost()` helpers; populate rates on TOKENS, Groq, OpenRouter
entries; add Ollama free-cost loop; add `CLAUDE_4_7_OPUS` to
`TOKEN_COST`.
- `frontend/.../NodeCost.tsx`: Refactor display logic into a typed
`CostDisplay` union; render in/out USD pair with cache-aware tooltip;
"Free" / "Pay-as-you-go" branches.
- `frontend/.../NodeCost.test.tsx`: Update existing cases for the new
copy; add coverage for token-rate pair (TOKENS + COST_USD), cache rates
in tooltip, "Free" label, and "Pay-as-you-go" label.
- `frontend/src/app/api/openapi.json`: Regenerated to include the new
optional fields.

### Checklist 📋

#### For code changes:
- [ ] I have clearly listed my changes in the PR description
- [ ] I have made a test plan
- [ ] I have tested my changes according to the test plan:
- [ ] Backend: `poetry run pytest
backend/executor/block_usage_cost_test.py
backend/executor/billing_reconciliation_test.py
backend/blocks/test/test_block.py` (1031 passed locally)
- [ ] Frontend: `npx vitest run .../NodeCost.test.tsx` (14 passed
locally)
- [ ] Manual: open AI Text Generator block in builder, verify Opus shows
`$5.00 in / $25.00 out per 1M tokens` with cached rates in tooltip; Qwen
3 Coder shows "Pay-as-you-go"; an Ollama model shows "Free"
- [ ] Manual: confirm post-flight billing for an OpenRouter call still
settles against `x-total-cost` (no regression — `_coerce_usd ×
cost_amount=150` path untouched)