Align reading statuses with MangaBaka, add auto-add sync, fix rating scale

[gregoryn22]

Summary

• Reading status rename: on_hold → paused across all layers (DB migration, backend validation, frontend dropdowns/pills/chips, CSS, export, sync maps). on_hold no longer exists anywhere.
• New statuses: considering fully wired in (was half-present); rereading added to all dropdowns and filter pills that were missing it.
• MB auto-add setting: new mb_auto_add toggle — when enabled, a series not found in the user's MB library during sync/push is added via POST /v1/my/library/{id} instead of silently skipped. Applies to live sync, import sync, and Push All.
• Rating scale bug fixed: app stores ratings 0–10 but MB expects 0–100. Push now multiplies ×10; pull now checks <= 100.0 and divides ÷10 with 0.5-step snap. Export also fixed. Was silently wrong before.
• MB series lifecycle: metadata refresh now handles merged (updates mb_linked_id, re-fetches with new ID) and deleted (skips with warning log).
• Pagination: pull_library() follows pagination.next URL instead of manual page math.
• Search limit: search() now sends limit=20 (was defaulting to 10).
• Scope check: test_mb_sync now returns missing_write_scope: true if the PAT lacks library.write; frontend shows a warning.

Files changed

File	What changed
app/database.py	Seed mb_auto_add=false; data migration on_hold → paused
app/mangabaka.py	search() limit param
app/mangabaka_sync.py	add_to_library() POST; fixed push_entry rating ×10; fixed pull_library pagination; considering in _STATE_MAP; paused replaces on_hold
app/notifier.py	Comment: "on hold" → "paused"
app/routers/export.py	_STATUS_MAP updated; rating export ×10 fix; considering added
app/routers/series.py	_VALID_READING_STATUSES updated; _bg_sync_to_mb + _bg_sync_import_to_mb check mb_auto_add
app/routers/settings.py	mb_auto_add in EXPOSED_KEYS + model; mb_pull rating fix; test_mb_sync scope check; push-all status exposes added count
app/scheduler.py	ACTIVE_STATUSES uses paused; push-all tracks added counter; merged/deleted series lifecycle in metadata refresh
static/css/components.css	.chip-hold → .chip-paused
static/index.html	All status dropdowns, filter pills, chips, help text updated; mb_auto_add toggle added to MB settings block
static/js/app.js	mb_auto_add default; push-all toast shows added count; test-sync toast warns on missing scope

Reviewer notes

• The DB migration (UPDATE tracked_series SET reading_status = 'paused' WHERE reading_status = 'on_hold') runs inside the existing engine.begin() block at startup — idempotent and safe on an already-migrated DB.
• add_to_library sends all progress fields in one POST body so no follow-up PATCH is needed.
• MB rating field is 0–100 integer; internal user_rating is 0–10 float. The ×10 / ÷10 conversion now happens at every boundary (push, pull, export).

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