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.

CHANGELOG

@@ -1,3 +1,18 @@
+cproof fork (unreleased)
+========================
+
+Changes:
+- Add flat-file database backend as alternative to SQLite for message history.
+  Stores messages as human-readable plain text files. Configure with `/privacy logging flatfile`.
+  Files are stored in ~/.local/share/profanity/flatlog/.
+- Add `/history verify [<jid>]` command to check integrity of stored message
+  history (works with both SQLite and flat-file backends).
+- Add vtable-based database backend abstraction allowing pluggable storage.
+- Add `make check-functional-flatfile` target to run functional tests with
+  flat-file backend.
+- Add AI client with multi-provider chat support (OpenAI, local, etc.).
+- Consolidate logging controls; show database backend in the statusbar.
+
 0.17.0 (2026-03-26)
 ===================