fix(review): address PR #94 reviewer comments

Blockers:
- SPDX/CProof copyright headers (7 files)
- stanza-id non-uniqueness: warn-only, don't skip (matches SQLite)
- fopen → open(O_EXCL|O_NOFOLLOW, 0600) + fdopen in export

Medium:
- log_info → log_warning for SQLite fallback
- remove prefs_save() auto-save from backend switch
- vtable null → log_warning in 4 dispatch functions
- log_debug → log_warning + null error handling in export
- rename failure → cons_show_error to user
- remove unused total_skipped variable
- LMC respects PREF_CORRECTION_ALLOW

Low nits:
- man page .SS fix + jabber.space URL
- profrc.example simplified
- show flatfile path on /logging flatfile
- EXPORT_PROGRESS_INTERVAL constant (magic 500)
- FF_META_PREFIX_ID/AID constants + FF_INDEX_STEP comment
- buffer overflow guard on close[1]/close[2]
- for-loop → memchr for JID resource scan
- BOM check extracted to ff_skip_bom() (DRY)
- index building extracted to _ff_maybe_index_line()
- O_APPEND+O_EXCL comment rewritten
- Makefile.am spaces → tabs

tests/unittests/test_database_export.c

@@ -1,12 +1,14 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+//
+// This file is part of CProof.
+// See LICENSE for the full GPLv3 text and the special OpenSSL linking exception.
+
 /*
  * test_database_export.c
  *
  * Unit tests for the flatfile write→parse round-trip, escape/unescape helpers,
  * jid_to_dir path construction, and parser edge cases — these are the core
  * functions exercised by database_export.c during export and import.
- *
- * Copyright (C) 2026 Profanity Contributors
- * License: GPL-3.0-or-later (same as Profanity)
  */
 
 #include "prof_cmocka.h"

tests/unittests/test_database_stress.c

@@ -1,12 +1,14 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+//
+// This file is part of CProof.
+// See LICENSE for the full GPLv3 text and the special OpenSSL linking exception.
+
 /*
  * test_database_stress.c
  *
  * Stress tests for flat-file database I/O: rapid-fire writes, MUC messages
  * with many participants, large message bodies, index/cache correctness,
  * export/import merge under load, and deep LMC correction chains.
- *
- * Copyright (C) 2026 Profanity Contributors
- * License: GPL-3.0-or-later (same as Profanity)
  */
 
 #include "prof_cmocka.h"