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

Makefile.am

@@ -195,9 +195,9 @@ functionaltest_sources = \
 	tests/functionaltests/test_disconnect.c tests/functionaltests/test_disconnect.h \
 	tests/functionaltests/test_history.c tests/functionaltests/test_history.h \
 	tests/functionaltests/test_lastactivity.c tests/functionaltests/test_lastactivity.h \
-        tests/functionaltests/test_autoping.c tests/functionaltests/test_autoping.h \
-        tests/functionaltests/test_disco.c tests/functionaltests/test_disco.h \
-        tests/functionaltests/test_export_import.c tests/functionaltests/test_export_import.h \
+	tests/functionaltests/test_autoping.c tests/functionaltests/test_autoping.h \
+	tests/functionaltests/test_disco.c tests/functionaltests/test_disco.h \
+	tests/functionaltests/test_export_import.c tests/functionaltests/test_export_import.h \
 	tests/functionaltests/functionaltests.c
 
 main_source = src/main.c