docs(saved-filters,dashboard,system,series): Beam WO3 batch 5 — 15 endpoints + dedupe

Beam drafted 15 frontmatter'd docs (saved-filters CRUD, dashboard
sections/preferences/restore, system config/settings, series list + books);
Claude verified shapes against handlers and pinned the registry:
- saved-filters: SavedFilterResponse pinned by type (RFC3339 timestamps here,
  unlike most of the API); create/update share CreateSavedFilterRequest
  (resource_type oneof media-items|collections|devices)
- dashboard: preferences GET pin {hidden_collections, collection_order,
  items_per_section}; PUT returns full UserDashboardPreferences row; sections
  wrapper {sections}
- system: GET+PUT /api/system/config are admin-defined key/value maps — new
  AnyJSONObject sentinel (gate shape-checks without key comparison; request
  side too). UpdateSystemConfiguration returns {status, message} — verified,
  not guessed. GET /api/system/settings returns []SettingEntry.
- series: {series[{name, library_id, book_count, total_in_series,
  cover_paths, last_entry_at}], total, limit, offset}; books by
  ?name=&library_id= with family-lens 403

Dedupe: deleted dashboard.md, system/config.md, system/settings.md — each
covered the same endpoints as Beam's per-endpoint files (content merged by
the gate-verified shapes; old prose recoverable from git).

Beam clarifications confirmed: scanner routes are WO5 (docs exist);
progress/books-match are WO5.

docs/developer/api/dashboard/get_sections.md

@@ -0,0 +1,49 @@
+---
+method: GET
+path: /api/dashboard/sections
+auth: jwt
+area: dashboard
+---
+<!-- source: internal/handlers/dashboard.go:28 DashboardHandler.GetSections -->
+
+# Get Dashboard Sections
+
+Returns all dashboard sections for a library, in the user's customized order.
+Respects `collection_order` and `hidden_collections` preferences. `library_id`
+is optional; when omitted the global (all-libraries) scope is used.
+
+## Example response
+
+```json example-response
+{
+  "sections": [
+    {
+      "id": "continue-reading",
+      "collection_id": "",
+      "is_system": true,
+      "title": "Continue Reading",
+      "description": "Books you're currently reading (0 < progress < 1)",
+      "icon": "📖",
+      "items": [],
+      "view_all_url": "",
+      "priority": 1,
+      "is_playlist": false,
+      "complete": false
+    }
+  ]
+}
+```
+
+## Query parameters
+
+| Param | Type | Required | Notes |
+| ----- | ---- | -------- | ----- |
+| library_id | uuid | No | Library to scope sections to |
+| limit | int | No | Items per section (1-100, default from preferences) |
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 400 | `{"error": "invalid library_id"}` | malformed library_id |
+| 500 | `{"error": "Failed to load dashboard sections"}` | unexpected server error |