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/algorithms.py

@@ -300,6 +300,7 @@ class SearchAlgorithm(ABC):
         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 search with the given parameters.
@@ -321,10 +322,13 @@ class SearchAlgorithm(ABC):
                 ``accessible_owners`` (ADR-027). ``None`` ⇒ open-ended.
             modified_before: Optional inclusive upper bound on ``modified_at``
                 (Unix seconds, UTC). ``None`` ⇒ open-ended.
-            path_prefix: Optional folder/path filter on the ``file_path`` payload
-                field (ADR-027 Phase 2). Only ``doc_type == "file"`` points carry
-                ``file_path``, so a non-empty value implicitly restricts results
-                to files. ``None`` ⇒ no path filter.
+            path_prefix: Deprecated single folder/path filter; folded into
+                ``path_prefixes``. Kept for backward compatibility.
+            path_prefixes: Optional folder/path filters on the ``file_path``
+                payload field (ADR-027 Phase 2), OR-ed together. Only
+                ``doc_type == "file"`` points carry ``file_path``, so any
+                non-empty value implicitly restricts results to files. ``None``
+                or empty ⇒ no path filter.
             **kwargs: Algorithm-specific parameters
 
         Returns: