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

profrc.example

@@ -49,12 +49,7 @@ grlog=true
 maxsize=1048580
 rotate=true
 shared=true
-# Database backend for message history:
-#   on       - SQLite database (default)
-#   off      - no message logging
-#   redact   - store with redacted message content
-#   flatfile - plain text files, editable with any text editor
-#              stored in ~/.local/share/profanity/flatlog/
+# Database backend: on (SQLite), off, redact, flatfile
 dblog=on
 
 [otr]