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

docs/profanity.1

@@ -197,7 +197,8 @@ Configuration for
 .B Profanity
 is stored in
 .I $XDG_CONFIG_HOME/profanity/profrc
-, details on commands for configuring Profanity can be found at <https://profanity-im.github.io/reference.html> or the respective built\-in help or man pages..SS Message History Storage
+, details on commands for configuring Profanity can be found at <https://jabber.space/command-reference/> or the respective built\-in help or man pages.
+.SS Message History Storage
 By default, message history is stored in an SQLite database. An alternative flat-file backend
 stores messages as human-readable plain text files that can be edited with any text editor.
 .PP