Migrate from sqlite3 to better-sqlite3

[hadv]

Summary

Migrate the backend database layer from sqlite3 to better-sqlite3 for improved performance and simpler API.

Motivation

Aspect	sqlite3	better-sqlite3
API	Async (callback/Promise)	Synchronous
Performance	Slower due to async overhead	2-10x faster for typical workloads
Simplicity	More complex with callbacks	Cleaner, simpler code
Transactions	Complex to handle properly	Easy: db.transaction(() => {...})()
Native builds	Often problematic	Generally more reliable
Maintenance	Less actively maintained	Actively maintained

Benefits for EthAura

• Passkey credential storage is I/O-bound locally - SQLite operations are fast enough that synchronous calls won't block Node.js meaningfully
• Simpler transaction handling - Passkey/guardian data operations need atomic transactions, which better-sqlite3 handles elegantly
• Better prepared statements - More intuitive API for parameterized queries (important for security)
• Fewer dependency issues - sqlite3 often has build problems with node-gyp

Tasks

• Install better-sqlite3 and remove sqlite3
• Update database initialization code
• Migrate all database queries to synchronous API
• Update transaction handling
• Test all passkey CRUD operations
• Test guardian/recovery operations
• Update any error handling for sync exceptions

References

• better-sqlite3 documentation