Cache the web card database in IndexedDB, gated on latest.json

[Manwe-777]

What

The web build downloads the ~5MB en-database.sqlite.gz from the Supabase mirror on every page load. latest.json is fetched, but only to check that the release publishes SQLite — it is never compared against anything, there is no local cache, and the mirror answers Cache-Control: no-cache so the browser's HTTP cache revalidates as well. The Electron build has always had the version-gated disk cache; the web path lost that behavior in the SQLite migration.

This gives the web the same flow against IndexedDB:

• The inflated database is stored in the existing mtgatool-local KV store under cardsDb:{lang} together with the version it holds.
• Each boot fetches only the tiny latest.json; the payload moves solely when the published version is newer than the cached one.
• Unreachable latest.json or a failed download serves the cached copy (a version behind beats no cards); a corrupt cache entry is ignored via the SQLite-header check; a broken IndexedDB falls through to the network, never blocking the cards.
• The worker source label distinguishes cache:vN from mirror:vN.

Verified

• Live against npm run start:web: after the first (seeding) load, a reload's only request to the Supabase bucket is latest.json — no .gz — and the worker boots from the cached bytes.
• New spec fetchDatabaseWeb.spec.ts covers the decision paths: up-to-date cache skips the payload entirely, offline falls back to the cache, no-cache-no-network yields null, corrupt cache is ignored.
• The transfer of dbBytes to the worker is safe with the cache: the structured clone into IndexedDB happens at kvPut before the buffer is transferred/detached.

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features

  • Added reliable web caching for card databases, reducing unnecessary downloads.
  • Uses cached data when release metadata, downloads, decompression, or validation are unavailable.
  • Records the database source and language-specific file details.

• Bug Fixes

  • Rejects invalid cached database files.
  • Gracefully returns no database when neither cached nor network data is available.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: eb655eab-58cc-468f-9e73-10e810779c10

📥 Commits

Reviewing files that changed from the base of the PR and between 4cd4bfd and 2e39f9f.

📒 Files selected for processing (3)

• src/utils/cardsDb/__tests__/fetchDatabaseWeb.spec.ts
• src/utils/cardsDb/fetchCardsDb.ts
• src/utils/cardsDb/loadCardsDbBytes.ts

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📝 Walkthrough

Walkthrough

Changes

Web database cache

Layer / File(s)	Summary
Cache storage and result contract src/utils/cardsDb/fetchCardsDb.ts	Added IndexedDB helpers that accept only valid SQLite bytes. Added the WebDatabase result shape with database bytes and source metadata.
Cached retrieval and mirror fallback src/utils/cardsDb/fetchCardsDb.ts, src/utils/cardsDb/__tests__/fetchDatabaseWeb.spec.ts	Updated fetchDatabaseWeb to use current cached data, handle metadata and mirror failures, validate downloads, persist successful data, and report its source. Added tests for these paths.
Database loader source propagation src/utils/cardsDb/loadCardsDbBytes.ts	Updated loadWeb to consume the new result shape and construct the language-specific source path.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Merge Risk: ⚪ Minimal · up to 2e39f

The change adds version-gated IndexedDB caching for the web card database with fallback behavior, and no actionable merge-blocking risk remains beyond normal checks and review.

Sequence Diagram(s)

sequenceDiagram
  participant loadWeb
  participant fetchDatabaseWeb
  participant IndexedDB
  participant ReleaseMetadata
  participant Mirror
  loadWeb->>fetchDatabaseWeb: Request database
  fetchDatabaseWeb->>IndexedDB: Read cached SQLite entry
  fetchDatabaseWeb->>ReleaseMetadata: Read release metadata
  fetchDatabaseWeb->>Mirror: Download and inflate payload
  Mirror-->>fetchDatabaseWeb: Return validated SQLite bytes
  fetchDatabaseWeb->>IndexedDB: Store bytes and release version
  fetchDatabaseWeb-->>loadWeb: Return bytes and source

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🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: caching the web card database in IndexedDB with latest.json version gating.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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