Speed up MAUI app sync (#208)

[fboucher]

Requested by Frank · project thread

Fixes #208

Before: every time the Posts page opened, the MAUI app downloaded all ~1500 posts, saved them one by one, made an extra HTTP call per unread post, and did all of it on the UI thread while re-rendering the progress bar for every post. The app froze during sync.

After: after the first sync, only posts changed since the last sync are downloaded and saved, all in one SQLite transaction. Sync runs off the UI thread, and the progress bar updates about every 250 ms. Posts deleted on the server are still removed locally, and HTML for read posts is still pruned.

How

• API: new GET api/posts/ids returns only post ids (read and unread). It projects just the key columns, so it stays small.
• Delta pull: PullAsync calls GetPostsModifiedAfter(lastSync ?? MinValue). Deletions are detected by comparing local posts with GetPostIds(). On the first sync the full list is already in hand, so the ids call is skipped. If the server is older and has no ids endpoint (404), sync falls back to the full list, so the app still works against an API that hasn't been redeployed yet.
• Batched writes: pulled posts are saved with SavePostsAsync, which runs in one transaction and preserves IsPendingSync. Server-deleted posts are removed with the new RemovePostsAsync in one transaction. The end state is the same as the old DeletePostAsync + MarkSyncedAsync pair. Duplicate rows for one post (the API joins posts with notes) are de-duplicated.
• Full post fetch: unread posts still get GetPost for the full details, but at most 5 requests run at once (Parallel.ForEachAsync).
• Threading: SyncService.SyncAsync starts the work with Task.Run, which covers every caller (Posts page, Storage page, app resume, connectivity change). All progress and conflict handlers already marshal with InvokeAsync.
• Progress throttling: pull and HTML-download progress events fire at most every 250 ms, plus the first and last events.
• SyncHtmlAsync logic is unchanged apart from the progress throttle.

Tests

• SyncServiceTests: existing pull tests updated for batched save. New tests cover delta pull with lastSync, deletion via ids, fallback when the ids endpoint is missing, first sync skipping the ids call, a 1500-post single-batch save, de-duplication, progress throttling, and bounded GetPost concurrency.
• LocalDataServiceTests: RemovePostsAsync, and SavePostsAsync preserving the pending-sync flag.
• DeltaApiTests: GET api/posts/ids returns read and unread posts and excludes deleted ones.
• Locally, NoteBookmark.MauiApp.Tests passes (57/57). The API tests build but weren't run here because they need Docker for Azurite, so CI covers them. The MAUI app project itself wasn't built locally (no MAUI workload); SyncService is compiled through the test project.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Kq68qA7Jv7puAF3Kc6GSFY

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[github-actions]

Package	Line Rate	Branch Rate	Health
NoteBookmark.AIServices	97%	64%	✔
NoteBookmark.Api	89%	80%	✔
NoteBookmark.BlazorApp	4%	10%	❌
NoteBookmark.Domain	93%	76%	✔
NoteBookmark.ServiceDefaults	96%	75%	✔
NoteBookmark.SharedUI	26%	33%	❌
Summary	58% (1164 / 2008)	50% (341 / 677)	❌