build(cmake): switch forum deep-index to FTS5, drop Xapian

Align the CMake build with the FTS5 forum content search already present
on this branch: compile deep_search/forumsindex_fts5.{cpp,hpp} instead of
the Xapian-based forumsindex.{cpp,hpp}, and remove the now-unused Xapian
find_package/link. The qmake build was already updated.

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

Author: jolavillette

CMakeLists.txt

@@ -578,9 +578,6 @@ endif(RS_JSON_API)
 ################################################################################
 
 if(RS_FORUM_DEEP_INDEX)
-	find_package(Xapian REQUIRED)
-	target_link_libraries(${PROJECT_NAME} PRIVATE ${XAPIAN_LIBRARIES})
-
 	target_compile_definitions(${PROJECT_NAME} PUBLIC RS_DEEP_FORUMS_INDEX)
 endif(RS_FORUM_DEEP_INDEX)
 

src/CMakeLists.txt

@@ -687,12 +687,12 @@ if(RS_FORUM_DEEP_INDEX)
 	list(
 		APPEND RS_SOURCES
 		deep_search/commonutils.cpp
-		deep_search/forumsindex.cpp )
+		deep_search/forumsindex_fts5.cpp )
 
 	list(
 		APPEND RS_IMPLEMENTATION_HEADERS
 		deep_search/commonutils.hpp
-		deep_search/forumsindex.hpp )
+		deep_search/forumsindex_fts5.hpp )
 endif(RS_FORUM_DEEP_INDEX)