fix: paginate tagged-folder SEARCH so the scanner discovers all files

The vector-sync scanner expanded a tagged folder into its PDF descendants via
`WebdavClient.find_by_type(scope=dir)` with no result limit. A WebDAV SEARCH
with no `<d:nresults>` returns only Nextcloud's default page (~100 on the
affected instance), so large tagged folders were silently truncated and most
documents were never queued for indexing (e.g. a 220-file folder yielded 100).

Add `search_files_all`, which pages the SEARCH to completion. It uses
`<d:firstresult>` offset paging where supported and, because Nextcloud 31
ignores offset (verified against a live instance), detects the repeated page
and falls back to a single bounded fetch with an explicit large `<d:nresults>`.
`find_all_by_type` wraps this and is now used for tagged-folder expansion;
`find_by_type` is unchanged for the interactive MCP tools.

Crossing `WEBDAV_SEARCH_MAX_RESULTS` logs a warning and increments the new
`astrolabe_document_scan_truncated_total` metric, so a coverage cap can never
again hide files silently.

Scope: this fixes discovery only. Cross-user double-processing of identical
shared files (point-ID collisions) is tracked separately.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/observability/metrics.py

@@ -278,6 +278,18 @@ documents_indexed_total = Counter(
     ["source", "status"],  # source: note | file | deck_card | news_item
 )
 
+# --- Document discovery / coverage ------------------------------------------
+#
+# Fires when a paged WebDAV SEARCH (folder-expansion during a scan) hits the
+# WEBDAV_SEARCH_MAX_RESULTS ceiling, meaning the discovered file set was capped
+# and some tagged documents may never be queued for indexing. This is the
+# alertable signal that prevents the old *silent* 100-result truncation from
+# recurring. Tenant is the Kubernetes ``namespace`` label, as elsewhere.
+document_scan_truncated_total = Counter(
+    "astrolabe_document_scan_truncated_total",
+    "Times a folder-expansion SEARCH hit the result ceiling (coverage truncated)",
+)
+
 # =============================================================================
 # Database Metrics
 # =============================================================================