feat: complete field parity, harden export/import, add tests

Export/Import improvements:
- Replace pagination with direct SQL query (db_sqlite_get_all_chat)
- Wrap import in SQL transaction with rollback on error
- Add fsync before fclose in export for data safety
- Sort merged output by timestamp with secondary key (stanza_id, from_jid)
- Export archive_id and marked_read from SQLite (lossless migration)
- Add progress indication every 500 messages during write/import
- Expand dedup key body prefix from 64 to 256 chars
- Fix g_slist_append O(n²) → g_slist_prepend + g_slist_reverse O(n)

Field parity (to_jid, to_resource, marked_read):
- Add fields to ff_parsed_line_t struct
- Write/parse to:|to_res:|read: metadata tags in flatfile format
- Pass to_resource through _ff_add_message and all callers
- Add marked_read to ProfMessage struct with -1 default (unset)
- Preserve fields across export/import round-trips

Tests (19 new: 11 unit + 8 functional):
- Unit: to_jid_and_marked_read, bracket_in_stanza_id, backslash_in_resource,
  mucpm_type, all_enc_types, crlf_handling, to_jid_special_chars,
  multiple_lines, parsed_line_free_null_safe, no_space_rejected,
  unclosed_bracket
- Functional: export_idempotent_no_duplicates, export_lmc_correction_survives,
  switch_preserves_old_backend_data, export_all_contacts,
  import_double_dedup, verify_after_export,
  switch_backends_independent_messages, export_empty_contact
- Rebalance test groups: move Chat Session from Group 3 to Group 4
  (25/33/30/27 instead of 25/33/36/21)
- Remove hardcoded test counts from group comments

Man page:
- Document /history switch sqlite|flatfile

docs/profanity.1

@@ -234,6 +234,10 @@ to copy messages from SQLite to flat-file format (merge with existing data), or
 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