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

src/database_flatfile.h

@@ -1,24 +1,12 @@
+// 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.
+
 /*
  * database_flatfile.h
  * vim: expandtab:ts=4:sts=4:sw=4
  *
- * Copyright (C) 2026 Profanity Contributors
- *
- * This file is part of Profanity.
- *
- * Profanity is free software: you can redistribute it and/or modify
- * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
- * (at your option) any later version.
- *
- * Profanity is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License
- * along with Profanity.  If not, see <https://www.gnu.org/licenses/>.
- *
  * Internal header shared between database_flatfile*.c modules.
  * Not part of the public API — do not include from outside the flatfile backend.
  */
@@ -69,8 +57,16 @@ typedef struct
 
 // --- Sparse index for single-file lookup ---
 
+// Sample every N-th log line for the sparse index.
+// 500 lines ≈ one index entry per ~50 KB of log data, so a 100K-message
+// contact requires only ~200 entries (~3 KB) for O(log n) time-range lookup.
 #define FF_INDEX_STEP 500
 
+// --- Metadata field prefixes (used in _ff_cache_line_ids and ff_parse_line) ---
+
+#define FF_META_PREFIX_ID       "id:"
+#define FF_META_PREFIX_AID      "aid:"
+
 typedef struct
 {
     off_t byte_offset;
@@ -127,6 +123,9 @@ char* ff_unescape_meta_value(const char* val);
 
 // --- I/O ---
 
+// Skip UTF-8 BOM if present; returns 3 if skipped, 0 otherwise.
+int ff_skip_bom(FILE* fp);
+
 char* ff_readline(FILE* fp, gboolean* truncated);
 void ff_write_line(FILE* fp, const char* timestamp, const char* type, const char* enc,
                    const char* stanza_id, const char* archive_id, const char* replace_id,