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>

backend/app/services/archive_extractor.py

@@ -16,9 +16,45 @@ log = logging.getLogger(__name__)
 
 ARCHIVE_EXTS = {".zip", ".cbz", ".rar", ".7z"}
 
+# Magic-byte signatures, so an archive with a mangled / extension-less filename
+# is still recognised. Patreon attachment download URLs sanitize to names like
+# `01_https___www.patreon.com_media-u_v3_131083093`, whose `Path.suffix` is junk
+# (`.com_media-u_v3_131083093`), never `.zip` — an extension-only gate filed
+# those as opaque PostAttachments and NEVER extracted them (operator-flagged
+# 2026-06-06). Detection is by extension first (cheap), then header sniff.
+_RAR_MAGIC = b"Rar!\x1a\x07"
+_7Z_MAGIC = b"7z\xbc\xaf\x27\x1c"
+
+
+def detect_archive_format(path: Path) -> str | None:
+    """Return "zip" | "rar" | "7z" for an archive, else None.
+
+    Trusts a known extension first, then falls back to magic-byte sniffing so a
+    mis-named or extension-less archive is still handled. (zip covers .cbz too.)
+    """
+    ext = Path(path).suffix.lower()
+    if ext in (".zip", ".cbz"):
+        return "zip"
+    if ext == ".rar":
+        return "rar"
+    if ext == ".7z":
+        return "7z"
+    try:
+        if zipfile.is_zipfile(path):
+            return "zip"
+        with open(path, "rb") as fh:
+            head = fh.read(8)
+    except OSError:
+        return None
+    if head.startswith(_RAR_MAGIC):
+        return "rar"
+    if head.startswith(_7Z_MAGIC):
+        return "7z"
+    return None
+
 
 def is_archive(path: Path) -> bool:
-    return Path(path).suffix.lower() in ARCHIVE_EXTS
+    return detect_archive_format(path) is not None
 
 
 @contextmanager
@@ -32,16 +68,16 @@ def extract_archive(path: Path):
     members: list[tuple[str, Path]] = []
     try:
         try:
-            ext = Path(path).suffix.lower()
-            if ext in (".zip", ".cbz"):
+            fmt = detect_archive_format(path)
+            if fmt == "zip":
                 with zipfile.ZipFile(path) as zf:
                     zf.extractall(base)
-            elif ext == ".rar":
+            elif fmt == "rar":
                 import rarfile
 
                 with rarfile.RarFile(path) as rf:
                     rf.extractall(base)
-            elif ext == ".7z":
+            elif fmt == "7z":
                 import py7zr
 
                 with py7zr.SevenZipFile(path, "r") as zf: