security: protect local data at rest (issue #146)

T03 — keep secrets out of profanity.log:
- new redact_secrets() helper (common.c) masks the content of SASL
  auth/response/challenge/success and <password> elements; applied in
  the libstrophe logger (_xmpp_file_logger) and the stderr-to-log
  bridge (REQ-DAR-03, REQ-LOG-06)
- _add_to_db() no longer logs decrypted message bodies or the full
  INSERT statement (REQ-DAR-03)

T07 — owner-only permissions on key material and history (REQ-AUTH-01):
- chmod 0600 on chatlog.db after open (journal/WAL files inherit)
- chmod 0600 on OTR keys.txt and fingerprints.txt after every write
- parent dirs are already 0700 (create_dir); this is defense in depth

T08 — integrity before trust:
- PRAGMA quick_check(1) gates every chatlog.db open; on failure the
  DB stays closed, the user is warned once in the console and the
  session continues without history (REQ-RES-02)
- OMEMO aesgcm downloads decrypt into a 0600 tempfile next to the
  target and rename it into place only after the GCM tag verifies;
  the open-command hook no longer runs on failed decryption
  (REQ-CRY-06)

Tests: redact_secrets unit tests; AES-256-GCM roundtrip and
tampered-tag/ciphertext unit tests (crypto.c now linked into
unittests under BUILD_OMEMO); functional test planting a corrupt
chatlog.db and asserting graceful degradation.

Closes #146

tests/unittests/unittests.c

@@ -48,6 +48,7 @@
 #include "test_ai_client.h"
 #include "test_database_export.h"
 #include "test_database_stress.h"
+#include "omemo/test_omemo_crypto.h"
 
 #define muc_unit_test(f) cmocka_unit_test_setup_teardown(f, muc_before_test, muc_after_test)
 
@@ -687,6 +688,12 @@ main(int argc, char* argv[])
         cmocka_unit_test(get_mentions__tests__various),
         cmocka_unit_test(release_is_new__tests__various),
         cmocka_unit_test(str_xml_sanitize__strips_illegal_characters),
+        cmocka_unit_test(redact_secrets__masks_credentials),
+#ifdef HAVE_OMEMO
+        cmocka_unit_test(aes256gcm_crypt_file__roundtrip_succeeds),
+        cmocka_unit_test(aes256gcm_crypt_file__rejects_tampered_tag),
+        cmocka_unit_test(aes256gcm_crypt_file__rejects_tampered_ciphertext),
+#endif
 
         cmocka_unit_test_setup_teardown(plugins_get_command_names__returns__no_commands,
                                         load_preferences,