feat: add server-internal database storage (skserver.sqlite)

[dirkwa]

Summary

Builds on #2409 (Database API). Adds getServerDb() so the server itself can persist operational data to a dedicated skserver.sqlite file — separate from plugin databases.

Motivated by the discussion in #2369 (review comment): tracking device token "last seen" timestamps needs persistence, and writing high-frequency data to security.json isn't the right tool.

• getServerDb() added as optional method on DatabaseProvider — existing community providers won't break
• Both built-in providers (node:sqlite, better-sqlite3) implement it
• Registry falls back to a built-in if the active community provider doesn't implement it
• Not exposed to plugins — server-internal only, not on the DatabaseApi interface
• Lazy: skserver.sqlite is only created when a consumer first calls getServerDb()
• Returns the same PluginDb handle (migrate, query, run, transaction)

File layout

{configPath}/ 
├── skserver.sqlite ← server-internal (NEW) 
├── plugin-db/
│          ├── my-plugin.db ← per-plugin (existing)
│          └── ... 
├── security.json 
└── settings.json

What this enables (future PRs)

Use case	Currently	With getServerDb()
Token "last seen"	Not tracked	Persist on each authenticated request
Access request audit trail	In-memory, lost on restart	Persist to database

Tests

Three new tests covering the key scenarios:

1. Registry fallback — registers a community provider without getServerDb(), verifies the registry falls back to the built-in provider and the server database works (migrate, insert, file exists)
2. Provider close() releases the database — opens the server db, stops the registry, opens a fresh registry on the same file, verifies no locked-file error
3. Type compatibility — a provider with only getPluginDb + close satisfies DatabaseProvider (no getServerDb required)

Tested

• Node 20 (better-sqlite3 only), Node 22, Node 24 — all pass
• Direct unit tests: migrate, insert, query, caching, file location, close
• Registry fallback logic verified
• Server startup: no skserver.sqlite created when no consumer calls it

Depends on: #2409

[dirkwa]

Further use cases:

In #2379:

• Show when a offline tagged device was last seen
• Optional: Store known devices cache (filebased atm)
• Optional: Store Source priority logic (filebased atm)

In BLE Provider API https://discord.com/channels/1170433917761892493/1478948783630057596:

• Show when offline tagged device was last seen
• Store known devices

[dirkwa]

Also worth looking into:

@signalk/resources-provider

• Stores waypoints, routes, notes, regions as one JSON file per resource
• Listing resources =readdir→ loop through every file → readFile + JSON.parse + stat each one
• No filtering at storage level — reads all, filters in JS
• A database would give indexed queries, pagination, spatial filtering