Add privacy-conscious course analytics

[mre]

Summary

• add a versioned SQLite event stream for chapter views, editor focus, hints, solution reveals, next-chapter navigation, and trusted server-side run outcomes
• capture test counts, run duration, and structured Rust error codes without storing source, names, URLs, user agents, or arbitrary metadata
• instrument the dashboard warm-up, quick tour, and exercise pages
• remove 195 obsolete or malformed submission rows through an explicit migration
• document the schema and useful analysis queries

Validation

• cargo test --locked --lib --bins
• cargo clippy --locked --lib --bins -- -D warnings
• cargo fmt --all --check
• ./scripts/check-examples.sh
• REQUIRE_COMPLETE=1 ./scripts/check-solutions.sh
• node --check static/js/analytics.js
• node --check static/js/inline-editor.js
• applied migration 011 to a production DB snapshot: 1491 -> 1296 submissions, with zero listed obsolete rows remaining