feat(draft): add /history export|import for SQLite<->flatfile migration

DRAFT — not yet tested end-to-end with a live XMPP session.

New commands:
  /history export [<jid>]  — copy messages from SQLite to flat-file
  /history import [<jid>]  — copy messages from flat-file to SQLite
Both merge with existing data; duplicates are skipped using stanza-id
or a SHA-256 fallback key (timestamp + sender + body prefix).

Implementation (src/database_export.c):
- Export: paginate SQLite via get_previous_chat, read existing flatfile
  for dedup, write merged result via ff_write_line + atomic rename
- Import: parse flatfile lines via ff_parse_line, build dedup set from
  SQLite, insert new messages via add_incoming (preserves original
  timestamps for both directions)
- List all contacts: db_sqlite_list_contacts() queries UNION of
  DISTINCT from_jid/to_jid; flatfile enumerates flatlog directories

Wiring:
- database.h: declare export/import functions + db_sqlite_list_contacts
- database_sqlite.c: add db_sqlite_list_contacts()
- cmd_defs.c: add export/import to /history synopsis and args
- cmd_funcs.c: add export/import handlers in cmd_history()
- cmd_ac.c: add 'export' and 'import' to history_ac
- Makefile.am: add database_export.c to core_sources
- stub_database.c: add stubs for test linking
- profanity.1: document export/import in man page

All code guarded with #ifdef HAVE_SQLITE — builds cleanly without it.

docs/profanity.1

@@ -226,6 +226,14 @@ Each line has the format:
 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.
 .SH BUGS
 Bugs can either be reported by raising an issue at the Github issue tracker:
 .br