Add optional user_id field to the progress collection. When a user is
authenticated, progress is keyed by user_id instead of session_id, making
it portable across devices and browsers. Anonymous reading still works via
session_id as before.
On login or registration, any progress accumulated under the anonymous
session is merged into the user's account (most-recent timestamp wins),
so chapters read before logging in are not lost.
- scripts/pb-init.sh: add user_id field to progress collection schema
- scraper/internal/storage/pocketbase.go: add user_id to EnsureCollections
- ui/src/lib/server/pocketbase.ts: Progress interface gains user_id;
getProgress/allProgress/setProgress accept optional userId and query/write
by user_id when present; add mergeSessionProgress() helper
- ui/src/routes/login/+page.server.ts: call mergeSessionProgress after
successful login and registration (fire-and-forget, non-fatal)
- ui/src/routes/api/progress/+server.ts: pass locals.user?.id to setProgress
- ui/src/routes/books/+page.server.ts: pass locals.user?.id to allProgress
- ui/src/routes/books/[slug]/+page.server.ts: pass locals.user?.id to getProgress
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