Security: Improve overall security in a few ways (#12501)

- Make sure we're always using regex with timeouts for user controlled data
- Adds rate limiting to the token endpoint (configurable)
- Signs the classifier pickle file with the SECRET_KEY and refuse to load one which doesn't verify.
- Require the user to set a secret key, instead of falling back to our old hard coded one

src/documents/regex.py

@@ -48,3 +48,73 @@ def safe_regex_search(pattern: str, text: str, *, flags: int = 0):
             textwrap.shorten(pattern, width=80, placeholder="…"),
         )
         return None
+
+
+def safe_regex_match(pattern: str, text: str, *, flags: int = 0):
+    """
+    Run a regex match with a timeout. Returns a match object or None.
+    Validation errors and timeouts are logged and treated as no match.
+    """
+
+    try:
+        validate_regex_pattern(pattern)
+        compiled = regex.compile(pattern, flags=flags)
+    except (regex.error, ValueError) as exc:
+        logger.error(
+            "Error while processing regular expression %s: %s",
+            textwrap.shorten(pattern, width=80, placeholder="…"),
+            exc,
+        )
+        return None
+
+    try:
+        return compiled.match(text, timeout=REGEX_TIMEOUT_SECONDS)
+    except TimeoutError:
+        logger.warning(
+            "Regular expression matching timed out for pattern %s",
+            textwrap.shorten(pattern, width=80, placeholder="…"),
+        )
+        return None
+
+
+def safe_regex_sub(pattern: str, repl: str, text: str, *, flags: int = 0) -> str | None:
+    """
+    Run a regex substitution with a timeout. Returns the substituted string,
+    or None on error/timeout.
+    """
+
+    try:
+        validate_regex_pattern(pattern)
+        compiled = regex.compile(pattern, flags=flags)
+    except (regex.error, ValueError) as exc:
+        logger.error(
+            "Error while processing regular expression %s: %s",
+            textwrap.shorten(pattern, width=80, placeholder="…"),
+            exc,
+        )
+        return None
+
+    try:
+        return compiled.sub(repl, text, timeout=REGEX_TIMEOUT_SECONDS)
+    except TimeoutError:
+        logger.warning(
+            "Regular expression substitution timed out for pattern %s",
+            textwrap.shorten(pattern, width=80, placeholder="…"),
+        )
+        return None
+
+
+def safe_regex_finditer(compiled_pattern: regex.Pattern, text: str):
+    """
+    Run regex finditer with a timeout. Yields match objects.
+    Stops iteration on timeout.
+    """
+
+    try:
+        yield from compiled_pattern.finditer(text, timeout=REGEX_TIMEOUT_SECONDS)
+    except TimeoutError:
+        logger.warning(
+            "Regular expression finditer timed out for pattern %s",
+            textwrap.shorten(compiled_pattern.pattern, width=80, placeholder="…"),
+        )
+        return