refactor: stabilize data routes and UI boundaries

[OXeu]

Summary

• add paginated feed data access, indexes, scoped cache operations, and batched tag writes
• introduce reusable route authorization and request-validation boundaries
• centralize UI theme tokens and modal behavior in @rin/ui
• add reusable API resource and resilient image handling
• fix long-text, image fallback, and mobile overflow issues
• unify client and server tests on bun:test, remove Vitest and jest-dom runner coupling, and align CI coverage

Validation

• bun run test
• bun run test:client
• bun run test:server
• bun run test:coverage (Bun 1.3.13, 434 passed)
• bun run check
• bun run format:check
• bun run build:client
• targeted ESLint for changed client test and config files
• git diff origin/main...HEAD --check

Notes

The existing repository-wide client lint baseline remains outside the scope of this refactor.

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