⚡ Bolt: Replace backend DB LEFT JOINs with Scalar Subqueries

[jspann21]

💡 What: Replaced LEFT JOINs paired with GROUP BY b.id patterns in get_library_books and get_hidden_books inside src-tauri/src/library/db.rs with embedded scalar subqueries.

🎯 Why: The original queries suffered from a significant N+1 Cartesian product issue, where joining arrays of tags and formats would multiply result rows exponentially before collapsing them using GROUP BY. SQLite evaluates these Cartesian products in-memory which acts as a major performance bottleneck for larger libraries.

📊 Impact: Significantly speeds up sorting, querying, and page load fetching since SQLite won't perform massive virtual joins to determine file sets or tags for single entries. We eliminate expensive group by allocations.

🔬 Measurement: Compile the backend (cargo test). Run pnpm dev with a large library (1,000+ books with multiple formats and tags). Measure the backend fetch times when scrolling or filtering the grid list. Wait times should be drastically reduced compared to main branch behavior.

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PR created automatically by Jules for task 1445289034428803278 started by @jspann21

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