perf(api): optimize static asset caching, enable ETag support, and expand response compression mime types.

[balazs-szucs]

Description of Changes

This PR implements a caching strategy for Stirling-PDF assets and API endpoints. It aligns the backend with modern SPA best practices, ensuring maximum performance for repeat visitors while guaranteeing immediate invalidation upon new deployments.

1. Granular Cache Tiering (WebMvcConfig.java)

• Vite-Hashed Assets (/assets/**): Configured as public, max-age=31536000, immutable. Since filenames are fingerprinted by Vite, these never need to be revalidated.
• Shell-Critical Metadata (/sw.js, /manifest.json): Configured as no-store to ensure the PWA lifecycle and atomic updates are handled safely by the browser.
• Branding Assets (Logos, Icons): Implemented Stale-While-Revalidate (max-age=1d, stale-while-revalidate=7d). This allows for immediate visual feedback while the browser revalidates branding changes in the background.
• Static Media & Fonts: Set to 1-year immutable caching to reduce redundant network requests for stable libraries like PDF.js.

2. API & SPA Freshness

• Global API Default: Updated EndpointInterceptor to set Cache-Control: no-cache for all /api/** routes. This forces browsers to revalidate dynamic data with the server (leveraging Last-Modified or ETags).
• SPA Entry Point: Updated ReactRoutingController to serve index.html with no-cache, ensuring users always receive the latest application bundle after a deployment.

3. Optimization & Cleanup

• Expanded Compression: Added image/svg+xml and modern font formats (woff2, ttf, otf, eot) to the server's compression MIME types.
• Reduced Overhead: Removed redundant VersionResourceResolver and ShallowEtagHeaderFilter. We now rely on Vite's native fingerprinting and Spring's built-in Last-Modified support, which avoids memory-intensive response buffering.

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Checklist

General

• I have read the Contribution Guidelines
• I have read the Stirling-PDF Developer Guide (if applicable)
• I have read the How to add new languages to Stirling-PDF (if applicable)
• I have performed a self-review of my own code
• My changes generate no new warnings

Documentation

• I have updated relevant docs on Stirling-PDF's doc repo (if functionality has heavily changed)
• I have read the section Add New Translation Tags (for new translation tags only)

Translations (if applicable)

• I ran scripts/counter_translation.py

UI Changes (if applicable)

• Screenshots or videos demonstrating the UI changes are attached (e.g., as comments or direct attachments in the PR)

Testing (if applicable)

• I have run task check to verify linters, typechecks, and tests pass
• I have tested my changes locally. Refer to the Testing Guide for more details.

[Copilot]

Pull request overview

This PR updates Spring MVC/static resource handling and response headers to implement a more explicit caching strategy for SPA assets and API responses, along with expanding server-side compression MIME types to improve repeat-visit performance.

Changes:

• Expanded server.compression.mime-types to include SVG and additional font MIME types.
• Updated ReactRoutingController to add Cache-Control: no-cache behavior when serving index.html.
• Reworked WebMvcConfig static resource handlers to apply tiered cache policies (immutable for hashed assets, no-store for PWA metadata, etc.) and added a global API cache header in EndpointInterceptor.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 6 comments.

File	Description
app/core/src/main/resources/application.properties	Expands compression MIME types for additional asset formats.
app/core/src/main/java/stirling/software/SPDF/controller/web/ReactRoutingController.java	Adds Cache-Control directives for SPA entry point responses.
app/core/src/main/java/stirling/software/SPDF/config/WebMvcConfig.java	Introduces tiered cache policies for different static asset groups.
app/core/src/main/java/stirling/software/SPDF/config/EndpointInterceptor.java	Sets a default Cache-Control header for /api/** responses.

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