Fix database lock issue: allow external access after sync finish

[meichuanyi]

Problem

The sync server holds an open database connection after sync operations complete, preventing external tools (like REST APIs, backup scripts, or monitoring tools) from accessing collection.anki2.

Root causes:

1. finish handler only clears sync_state but doesn't close the Collection
2. server_finish doesn't execute WAL checkpoint
3. SQLite busy_timeout=0 causes immediate failure on lock contention

Solution

Three minimal changes:

1. Close Collection after finish (handlers.rs)

user.sync_state = None;
user.col = None; // Close collection to release database lock

2. Force WAL checkpoint on finish (finish.rs)

col.storage.checkpoint()?; // Force WAL checkpoint to release database lock

3. Set reasonable busy_timeout (sqlite.rs)

db.busy_timeout(std::time::Duration::from_secs(5))?; // Allow 5 seconds for lock contention

Testing

• ✅ Compiled successfully with Rust 1.92.0
• ✅ Health check passes
• ✅ Database accessible during sync
• ✅ Database accessible after finish

Impact

• Minimal code changes (3 files, 4 lines)
• No change to sync protocol or client behavior
• Allows external tools to access database safely after sync completes
• busy_timeout=5s provides graceful handling for brief lock contention

Use Case

This enables scenarios like:

• REST APIs for AI + Anki integration
• Automated backup scripts
• Database monitoring tools
• External card import/export tools

All can now operate without conflicting with sync server's database connection.