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_config.md

@@ -0,0 +1,30 @@
+---
+method: GET
+path: /api/system/config
+auth: jwt+admin
+area: system
+---
+<!-- source: internal/handlers/sidecar.go:374 SidecarHandler.GetSystemConfiguration -->
+
+# Get System Configuration
+
+Returns all system-wide configuration key/value pairs (backed by the
+`system_config` table).
+
+## Example response
+
+```json example-response
+{
+  "base_url": "https://books.example.com",
+  "opds_base_url": "https://books.example.com/opds",
+  "api_base_url": "https://books.example.com/api"
+}
+```
+
+UNVERIFIED: exact config key set is arbitrary (admin-defined); example shows common keys only.
+
+## Errors
+
+| Status | Body | When |
+| ------ | ---- | ---- |
+| 500 | `{"error": "failed to fetch system config"}` | unexpected server error |