Add add-database and remove-database lifecycle events

[asg017]

Adds two new plugin-visible events that fire when a database is attached to or detached from a running Datasette instance:

• add-database — fired by Datasette.add_database() (and therefore add_memory_database())
• remove-database — fired by Datasette.remove_database()

Both events carry database (the attached name), path (resolved absolute filesystem path, None for in-memory databases) and is_memory. actor is None for programmatic calls.

Motivation

Plugins that manage databases have no way to learn about databases added or removed after startup. The motivating consumer is datasette-litestream, which runs a long-lived replication daemon: any database attached at runtime (e.g. by datasette-upload-dbs) is silently not replicated — a data-durability gap. With these events it can register new databases with the daemon immediately, and trigger a final sync on removal.

Delivery semantics

• Best-effort, fire-and-forget: dispatch happens via loop.create_task() (with strong references held until completion, so tasks can't be garbage-collected mid-flight). Listeners run shortly after the change, not before the method returns.
• Runtime-only: events fire only after invoke_startup() has completed and while an event loop is running. Databases attached during startup fire nothing — plugins that need those can iterate datasette.databases in their own startup hook. This also avoids the track_event() assertion failing before event classes are registered.
• remove-database fires after close(): close() drains queued writes first, so a listener doing a final read of the file sees everything. The database file is never deleted (the exception is temp-disk databases, which delete their backing file on close — documented).
• Rapid successive changes to the same name may reach listeners interleaved; documented as something listeners must tolerate.

Changes

• datasette/events.py — new AddDatabaseEvent / RemoveDatabaseEvent dataclasses, registered in the core register_events() hookimpl
• datasette/app.py — _track_event_soon() helper + dispatch from add_database() / remove_database()
• docs/events.md, docs/internals.rst — event docs and the delivery contract
• Tests: registration, file/memory databases, remove (file untouched on disk), pre-startup and no-running-loop no-ops, auto-renamed names in events

Out of scope (possible follow-ups)

• HTTP API for attaching/detaching databases (permissions, allowlisted directory, persistence story)
• Any file deletion on removal — removal is detach-only

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 0% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (e889403) to head (c6bbcc3).

Files with missing lines	Patch %	Lines
datasette/app.py	0.00%	18 Missing ⚠️
datasette/events.py	0.00%	12 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2870   +/-   ##
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  Coverage   0.00%   0.00%           
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  Files         73      73           
  Lines      12279   12307   +28     
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- Misses     12279   12307   +28     

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