Add stats table

[chris124567]

Close #182

[Copilot]

Pull request overview

This PR introduces an incrementally maintained stats table in the SQLite persistence layer to make UploadStats queries scale better on larger databases (per Issue #182), replacing the prior full-table aggregation approach.

Changes:

• Added a stats table plus helpers to increment/decrement upload-pipeline counters transactionally.
• Updated object + multipart upload write paths to maintain stats (pending/uploaded/orphaned/multipart counts and sizes) as state changes occur.
• Added schema init + migration backfill logic, and expanded TestUploadStats to cover additional state transitions.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
sia/persist/sqlite/stats.go	Introduces stat keys, incremental update helpers, and the new Store.UploadStats() implementation backed by the stats table.
sia/persist/sqlite/objects.go	Updates object lifecycle operations to adjust stats and moves UploadStats out of this file.
sia/persist/sqlite/objects_test.go	Extends TestUploadStats to validate stat counter behavior across more transitions.
sia/persist/sqlite/multipart.go	Updates multipart create/complete/abort flows to maintain multipart_uploads and pending-object stats.
sia/persist/sqlite/migrations.go	Adds migration to create/backfill the stats table for existing DBs.
sia/persist/sqlite/init.sql	Adds stats table creation and initialization for fresh DBs.

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