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.

src/command/cmd_defs.c

@@ -1882,14 +1882,20 @@ static const struct cmd_t command_defs[] = {
               CMD_TAG_CHAT)
       CMD_SYN(
               "/history on|off",
-              "/history verify [<jid>]")
+              "/history verify [<jid>]",
+              "/history export [<jid>]",
+              "/history import [<jid>]")
       CMD_DESC(
               "Switch chat history on or off, /logging chat will automatically be enabled when this setting is on. "
               "When history is enabled, previous messages are shown in chat windows. "
-              "Use 'verify' to check integrity of stored message history.")
+              "Use 'verify' to check integrity of stored message history. "
+              "Use 'export' to copy messages from SQLite to flat-file format, or 'import' to copy from flat-file to SQLite. "
+              "Both export and import merge with existing data (duplicates are skipped).")
       CMD_ARGS(
               { "on|off", "Enable or disable showing chat history." },
-              { "verify [<jid>]", "Verify integrity of message history. Optionally specify a JID to check only one contact." })
+              { "verify [<jid>]", "Verify integrity of message history. Optionally specify a JID to check only one contact." },
+              { "export [<jid>]", "Export SQLite history to flat-file format. Optionally specify a JID to export only one contact." },
+              { "import [<jid>]", "Import flat-file history into SQLite. Optionally specify a JID to import only one contact." })
     },
 
     { CMD_PREAMBLE("/log",