fix(archive): magic-byte archive detection so mis-named archives extract — #713 part 1

Patreon attachment downloads land with sanitized URL-blob filenames
(01_https___www.patreon.com_media-u_v3_<id>) whose Path.suffix is junk, never
.zip — so the extension-only is_archive() filed them as opaque PostAttachments
and never extracted them ("No images attached to this post").

Add archive_extractor.detect_archive_format() — extension first, then magic-byte
sniff (zipfile.is_zipfile + RAR/7z signatures). is_archive(), extract_archive(),
and safe_probe._inspect_archive() (the bomb-guard) all route through it, so a
mis-named/extension-less archive is now detected, bomb-guarded, integrity-tested,
AND extracted regardless of filename. Stops new ones; part 2 re-extracts the
already-imported backlog.

Tests: mis-named zip detected + extracted; non-archive dotted name not
misdetected; _inspect_archive on a mis-named zip; signature updated.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_safe_probe.py

@@ -40,11 +40,21 @@ def test_probe_archive_corrupt_zip_clean_rejection(tmp_path):
 def test_inspect_archive_reports_size_and_clean_integrity(tmp_path):
     z = tmp_path / "sized.zip"
     _zip(z, {"a.txt": b"x" * 100, "b.txt": b"y" * 50})
-    total, bad = safe_probe._inspect_archive(z, ".zip")
+    total, bad = safe_probe._inspect_archive(z)
     assert total == 150
     assert bad is None
 
 
+def test_inspect_archive_detects_misnamed_zip(tmp_path):
+    """A zip with a mangled / extension-less name (Patreon URL-blob attachment)
+    is still bomb-guarded + integrity-tested via magic-byte detection."""
+    z = tmp_path / "01_https___www.patreon.com_media-u_v3_131083093"
+    _zip(z, {"a.txt": b"x" * 100})
+    total, bad = safe_probe._inspect_archive(z)
+    assert total == 100
+    assert bad is None
+
+
 def test_run_probe_bomb_guard(tmp_path, monkeypatch):
     """In-process call to _run_probe so the monkeypatched cap takes effect.
     The public probe_archive runs in a subprocess which re-imports the