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/system/get_settings.md

@@ -0,0 +1,34 @@
+---
+method: GET
+path: /api/system/settings
+auth: jwt+admin
+area: system
+---
+<!-- source: internal/handlers/system_settings.go:44 SystemSettingsHandler.GetSettings -->
+
+# Get System Settings
+
+Returns metadata and current values for every known tunable system setting.
+
+## Example response
+
+```json example-response
+[
+  {
+    "key": "scan_poll_interval_seconds",
+    "value": "60",
+    "type": "int",
+    "requires_restart": false,
+    "category": "scanner",
+    "group": "Scanning",
+    "description": "How often to scan all libraries (seconds)",
+    "is_default": true
+  }
+]
+```
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 503 | `{"error": "settings registry not initialized"}` | registry not wired |