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.

docs/profanity.1

@@ -205,7 +205,48 @@ Configuration for
 .B Profanity
 is stored in
 .I $XDG_CONFIG_HOME/profanity/profrc
-, details on commands for configuring Profanity can be found at <https://profanity-im.github.io/reference.html> or the respective built\-in help or man pages.
+, details on commands for configuring Profanity can be found at <https://jabber.space/command-reference/> or the respective built\-in help or man pages.
+.SS Message History Storage
+By default, message history is stored in an SQLite database. An alternative flat-file backend
+stores messages as human-readable plain text files that can be edited with any text editor.
+.PP
+To enable flat-file logging, set in
+.IR profrc :
+.PP
+.EX
+[logging]
+dblog=flatfile
+.EE
+.PP
+Or use the command:
+.B /privacy logging flatfile
+.PP
+Flat-file logs are stored under
+.IR $XDG_DATA_HOME/profanity/flatlog/ ,
+organized as
+.IR {account_jid}/{contact_jid}/history.log .
+.PP
+Each line has the format:
+.br
+.EX
+{ISO8601} [{type}|{enc}|id:{id}|aid:{archive_id}|corrects:{id}] {sender}: {message}
+.EE
+.PP
+Use
+.B /history verify
+to check integrity of stored history (both SQLite and flat-file).
+.PP
+Use
+.B /history export [<jid>]
+to copy messages from SQLite to flat-file format (merge with existing data), or
+.B /history import [<jid>]
+to copy from flat-file to SQLite.
+Both operations skip duplicates.
+Omit the JID to process all contacts.
+.PP
+Use
+.B /history switch sqlite|flatfile
+to change the active database backend at runtime without reconnecting.
 .SH BUGS
 Bugs can either be reported by raising an issue at the Github issue tracker:
 .br