fix(consistency): align entry fetch content behavior between API and UI

[AiraNadih]

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• I have tested my changes
• There are no breaking changes
• I have thoroughly tested my changes and verified there are no regressions
• My commit messages follow the Conventional Commits specification
• I have read this document: https://miniflux.app/faq.html#pull-request

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Problem

I noticed a behavioral inconsistency between the UI and API when working with entries:

When fetching content via the API endpoint, the response was returning raw entry content without:

1. Updating the entry title and content (which happens in the UI flow)
2. Rewriting document URLs with relative proxy URLs

This inconsistency creates different behaviors depending on whether users interact through the UI or API.

Solution

This PR aligns the API behavior with the UI by:

• Adding a call to UpdateEntryTitleAndContent() when fetching content via API
• Using mediaproxy.RewriteDocumentWithRelativeProxyURL() to ensure consistent URL handling

These changes ensure that both the API and UI provide the same functionality and data format when fetching entry content.

Related Behavior

This fix is similar to an issue I noticed previously: when modifying a feed through the web form (e.g., disabling it), the parsing error count and error message are reset. However, this reset doesn't occur when modifying via API. This PR addresses a similar type of UI/API inconsistency.

[fguillot]

Updating the entry title and content is considered a breaking change, as it alters the API contract. Previously, this endpoint was read-only, so users will experience different behavior after upgrading Miniflux.

I recommend gating this change behind a feature flag (query-string parameter) or introducing a new endpoint to preserve backward compatibility.