Create a cache eviction policy for localforage

What is this about?

@davidmurdoch found a localforage cache in shared/lib/storage-helpers.js that's used in shared/lib/fetch-with-cache.ts that doesn't seem to ever evict cached entries. We want to make sure that cached items are removed when no longer needed.
Docs: https://localforage.github.io/localForage/#data-api

Part of this work will be to write a migration to clear out all pre existing obsolete cache entries.

Scenario

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Design

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Technical Details

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Threat Modeling Framework

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Acceptance Criteria

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Stakeholder review needed before the work gets merged

• Engineering (needed in most cases)
• Design
• Product
• QA (automation tests are required to pass before merging PRs but not all changes are covered by automation tests - please review if QA is needed beyond automation tests)
• Security
• Legal
• Marketing
• Management (please specify)
• Other (please specify)

References

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Start by reading shared/lib/storage-helpers.js and shared/lib/fetch-with-cache.ts, then consult the localForage Data API documentation. Define the cache eviction behavior and migration scope from the existing cache usage. Done means obsolete pre-existing entries are cleared and cached items are removed when no longer needed.