dx(backend): Make Builder and Marketplace search work without embeddings (#12479)

When OpenAI credentials are unavailable (fork PRs, dev envs without API
keys), both builder block search and store agent functionality break:

1. **Block search returns wrong results.** `unified_hybrid_search` falls
back to a zero vector when embedding generation fails. With ~200 blocks
in `UnifiedContentEmbedding`, the zero-vector semantic scores are
garbage, and lexical matching on short block names is too weak — "Store
Value" doesn't appear in the top results for query "Store Value".

2. **Store submission approval fails entirely.**
`review_store_submission` calls `ensure_embedding()` inside a
transaction. When it throws, the entire transaction rolls back — no
store submissions get approved, the `StoreAgent` materialized view stays
empty, and all marketplace e2e tests fail.

3. **Store search returns nothing.** Even when store data exists,
`hybrid_search` queries `UnifiedContentEmbedding` which has no store
agent rows (backfill failed). It succeeds with zero results rather than
throwing, so the existing exception-based fallback never triggers.

### Changes 🏗️

- Replace `unified_hybrid_search` with in-memory text search in
`_hybrid_search_blocks` (-> `_text_search_blocks`). All ~200 blocks are
already loaded in memory, and `_score_primary_fields` provides correct
deterministic text relevance scoring against block name, description,
and input schema field descriptions — the same rich text the embedding
pipeline uses. CamelCase block names are split via `split_camelcase()`
to match the tokenization from PR #12400.

- Make embedding generation in `review_store_submission` best-effort:
catch failures and log a warning instead of rolling back the approval
transaction. The backfill scheduler retries later when credentials
become available.

- Fall through to direct DB search when `hybrid_search` returns empty
results (not just when it throws). The fallback uses ad-hoc
`to_tsvector`/`plainto_tsquery` with `ts_rank_cd` ranking on
`StoreAgent` view fields, restoring the search quality of the original
pre-hybrid implementation (stemming, stop-word removal, relevance
ranking).

- Fix Playwright artifact upload in end-to-end test CI

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - [x] `build.spec.ts`: 8/8 pass locally (was 0/7 before fix)
  - [x] All 79 e2e tests pass in CI (was 15 failures before fix)

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Co-authored-by: Reinier van der Leer (@Pwuts)

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Co-authored-by: Reinier van der Leer <[email protected]>
Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>