feat(search): support multiple folders in the semantic-search path filter

Extend the ADR-027 Phase 2 path filter from a single path_prefix to a
list of folders. The new normalize_path_prefixes() helper is the single
source of truth for trimming, dropping blanks, and de-duplicating, and
folds the legacy single path_prefix into the list for backward
compatibility.

build_base_filter_conditions() adds one MatchText to the must clause for
a single folder (unchanged shape) and OR-s multiple folders via a nested
Filter(should=[...]) so a file under any selected folder matches while
still AND-ing against the ACL/doc_type/date conditions.

path_prefixes is threaded through every search surface: the
nc_semantic_search MCP tool, the visualization API (JSON body), and the
viz route (CSV query param). The Astrolabe frontend folder picker that
produces these lists ships in a companion astrolabe PR.

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

nextcloud_mcp_server/search/semantic.py

@@ -55,6 +55,7 @@ class SemanticSearchAlgorithm(SearchAlgorithm):
         modified_after: int | None = None,
         modified_before: int | None = None,
         path_prefix: str | None = None,
+        path_prefixes: list[str] | None = None,
         **kwargs: Any,
     ) -> list[SearchResult]:
         """Execute semantic search using vector similarity.
@@ -79,8 +80,11 @@ class SemanticSearchAlgorithm(SearchAlgorithm):
                 seconds, UTC); ``None`` ⇒ open-ended (ADR-027).
             modified_before: Inclusive upper bound on ``modified_at`` (Unix
                 seconds, UTC); ``None`` ⇒ open-ended (ADR-027).
-            path_prefix: Folder/path filter on ``file_path`` (files only);
-                ``None`` ⇒ no path filter (ADR-027 Phase 2).
+            path_prefix: Deprecated single folder filter; folded into
+                ``path_prefixes`` (ADR-027 Phase 2).
+            path_prefixes: Folder/path filters on ``file_path`` (files only),
+                OR-ed together; ``None``/empty ⇒ no path filter (ADR-027
+                Phase 2).
             **kwargs:
                 - score_threshold (float): override the instance default
 
@@ -122,6 +126,7 @@ class SemanticSearchAlgorithm(SearchAlgorithm):
             modified_after=modified_after,
             modified_before=modified_before,
             path_prefix=path_prefix,
+            path_prefixes=path_prefixes,
         )
 
         # ACL pre-filter (design §11), opt-in via ACL_PREFILTER_ENABLED and OFF