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_preferences.md

@@ -0,0 +1,34 @@
+---
+method: GET
+path: /api/dashboard/preferences
+auth: jwt
+area: dashboard
+---
+<!-- source: internal/handlers/dashboard.go:285 DashboardHandler.GetPreferences -->
+
+# Get Dashboard Preferences
+
+Returns the signed-in user's dashboard preferences for a library (or the
+global scope when `library_id` is omitted).
+
+## Example response
+
+```json example-response
+{
+  "hidden_collections": [],
+  "collection_order": [],
+  "items_per_section": 20
+}
+```
+
+## Query parameters
+
+| Param | Type | Required | Notes |
+| ----- | ---- | -------- | ----- |
+| library_id | uuid | No | Library scope; omitted = global |
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 400 | `{"error": "invalid library_id"}` | malformed library_id |