Add typed content collections

[owjs3901]

Summary

• add compile-time typed content collection declarations and concrete Rust request/response handlers
• persist shared draft/published entries with collection-scoped slugs, author/timestamps, and transactional audit events
• export schemas/defaults for reusable generated collection hub, list, editor, filters, and pagination UI
• add the reference Content pages collection with canonical Media reference validation

Full gate

• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace
• RUSTDOCFLAGS=-D warnings cargo doc --workspace --no-deps
• bunx oxlint
• bun x tsc --noEmit in packages/app, apps/example-app, and every plugin workspace
• cargo run -p yeollin-cli -- build --skip-backend from apps/example-app (18 routes prerendered)

Browser QA

• signed in through the real vinext dev UI
• uploaded a PNG in Media and used its stable media reference in Content
• verified collection hub, empty state, create, edit, status filter, publish, exact public retrieval, unpublish, and delete
• verified drafts return 404 from the public endpoint and the browser reported no page errors