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.c

@@ -75,7 +75,7 @@ log_database_init(ProfAccount* account)
         active_db_backend = db_backend_sqlite();
         log_info("Using SQLite database backend");
 #else
-        log_info("SQLite not compiled in, falling back to flat-file backend");
+        log_warning("SQLite not compiled in, falling back to flat-file backend");
         active_db_backend = db_backend_flatfile();
 #endif
     }
@@ -103,9 +103,8 @@ log_database_switch_backend(const char* new_backend)
     // Close current backend
     log_database_close();
 
-    // Update preference
+    // Update preference (user must /save to persist across restarts)
     prefs_set_string(PREF_DBLOG, new_backend);
-    prefs_save();
 
     // Get current account to reinitialize
     const char* account_name = session_get_account_name();
@@ -130,6 +129,8 @@ log_database_add_incoming(ProfMessage* message)
 {
     if (active_db_backend && active_db_backend->add_incoming) {
         active_db_backend->add_incoming(message);
+    } else {
+        log_warning("log_database_add_incoming: no backend or method available");
     }
 }
 
@@ -138,6 +139,8 @@ log_database_add_outgoing_chat(const char* const id, const char* const barejid,
 {
     if (active_db_backend && active_db_backend->add_outgoing_chat) {
         active_db_backend->add_outgoing_chat(id, barejid, message, replace_id, enc);
+    } else {
+        log_warning("log_database_add_outgoing_chat: no backend or method available");
     }
 }
 
@@ -146,6 +149,8 @@ log_database_add_outgoing_muc(const char* const id, const char* const barejid, c
 {
     if (active_db_backend && active_db_backend->add_outgoing_muc) {
         active_db_backend->add_outgoing_muc(id, barejid, message, replace_id, enc);
+    } else {
+        log_warning("log_database_add_outgoing_muc: no backend or method available");
     }
 }
 
@@ -154,6 +159,8 @@ log_database_add_outgoing_muc_pm(const char* const id, const char* const barejid
 {
     if (active_db_backend && active_db_backend->add_outgoing_muc_pm) {
         active_db_backend->add_outgoing_muc_pm(id, barejid, message, replace_id, enc);
+    } else {
+        log_warning("log_database_add_outgoing_muc_pm: no backend or method available");
     }
 }