Merge branch 'master' into merge/upstream-full

Resolve conflicts after master gained the flat-file database backend,
AI client, and other cproof-fork features:

- CHANGELOG: keep cproof-fork unreleased section atop full upstream
  history (0.17.0 / 0.16.0 / 0.15.1)
- src/command/cmd_ac.c: keep both upstream spellcheck AC and cproof
  /history switch|verify|export|import autocompletion
- src/config/preferences.c: merge upstream [spellcheck] group with the
  cproof [ai]/[ai/<provider>] groups
- src/database.c: take master (thin dispatcher); upstream SQLite
  improvements (v3 migration, auto_gchar) belong in database_sqlite.c
  and will land in a follow-up commit
- src/tools/autocomplete.c: take upstream unescape of search_str
- src/ui/statusbar.c: keep guint signatures (per project preference)
  and adapt _status_bar_draw_dbbackend to guint as well
- src/xmpp/jid.c: combine the new jid_is_valid/jid_is_valid_user_jid
  with the "(null)" legacy-placeholder guard in
  jid_create_from_bare_and_resource; use auto_gchar
- tests/functionaltests/proftest.c: keep unsigned-safe pre-check on
  output_len before subtracting OUTPUT_BUF_SIZE
- tests/unittests/unittests.c: keep upstream subdirectory include
  layout plus cproof-only test_ai_client/database_export/database_stress
- tests/unittests/test_jid.c: drop orphan top-level copy (the
  restructured version lives in tests/unittests/xmpp/test_jid.c)

Upstream is included up to 1ac05754b (release 0.18.0 era). Follow-up:
pull 1ac05754b..upstream/master (esp. 42a849d16 db migration fix and
4749645c7 receipt-request restore), and adapt the SQLite improvements
into database_sqlite.c.

tests/unittests/database/stub_database.c

@@ -9,7 +9,15 @@
 #include <glib.h>
 #include "prof_cmocka.h"
 
+#include "config.h"
 #include "database.h"
+#include "database_flatfile.h"
+
+db_backend_t* active_db_backend = NULL;
+
+// The flatfile parser needs this global for path construction.
+// In tests we keep it NULL since no real file I/O happens via the backend.
+char* g_flatfile_account_jid = NULL;
 
 gboolean
 log_database_init(ProfAccount* account)
@@ -36,3 +44,66 @@ void
 log_database_close(void)
 {
 }
+gboolean
+log_database_switch_backend(const char* new_backend)
+{
+    return TRUE;
+}
+GSList*
+log_database_verify_integrity(const gchar* const contact_barejid)
+{
+    return NULL;
+}
+void
+integrity_issue_free(integrity_issue_t* issue)
+{
+    if (issue) {
+        g_free(issue->file);
+        g_free(issue->message);
+        g_free(issue);
+    }
+}
+#ifdef HAVE_SQLITE
+db_backend_t*
+db_backend_sqlite(void)
+{
+    return NULL;
+}
+GSList*
+db_sqlite_list_contacts(void)
+{
+    return NULL;
+}
+GSList*
+db_sqlite_get_all_chat(const gchar* const contact_barejid)
+{
+    return NULL;
+}
+void
+db_sqlite_begin_transaction(void)
+{
+}
+void
+db_sqlite_end_transaction(void)
+{
+}
+void
+db_sqlite_rollback_transaction(void)
+{
+}
+int
+log_database_export_to_flatfile(const gchar* const contact_jid)
+{
+    return 0;
+}
+int
+log_database_import_from_flatfile(const gchar* const contact_jid)
+{
+    return 0;
+}
+#endif
+db_backend_t*
+db_backend_flatfile(void)
+{
+    return NULL;
+}