Phase 8: Add transactional content search

[owjs3901]

Summary

• add the search plugin with a SQLite FTS5 external-content index, startup backfill/rebuild, and stale-content cleanup
• keep create, update, publish, unpublish, and delete projections in the originating content transaction through one Inline subscriber
• expose administrator-only, title-weighted, paginated /api/search with bounded operator-safe prefix queries plus collection/status filters
• ship the authenticated /search UI with loading, error, forbidden, empty, result, and pagination states
• make the authenticated shell responsive after mobile QA exposed the fixed sidebar overflow
• document the index model, authorization boundary, query contract, and write-synchronization invariant

Validation

• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace
• $env:RUSTDOCFLAGS='-D warnings'; cargo doc --workspace --no-deps
• bunx oxlint (passes; retains one pre-existing non-fatal plugins/auth hook advisory)
• bun x tsc --noEmit in packages/app, apps/example-app, and every plugin with a tsconfig.json
• cargo run -p yeollin-cli -- build --skip-backend from apps/example-app (20 routes prerendered, including /search)

Browser QA

• signed in through the real UI and created two content entries
• verified partial-prefix search, title-before-body ranking, draft/published badges, empty published filtering, immediate publish projection, and result navigation on desktop
• repeated search on an emulated iPhone 12 and verified both result cards, the horizontally scrollable navigation, and zero document-level horizontal overflow
• application console had no application-origin errors; Orca device emulation emitted only its URL-less webdriver injection diagnostic
• restored the original example-app database byte-for-byte after QA