Exempt task webhook trigger from session auth (#784)

POSTing to the per-task webhook URL shown in the Tasks UI returned 401
Unauthorized even though the URL is labelled "no auth needed". The
trigger handler at routes/task_routes.py:873 (`POST
/api/tasks/{task_id}/webhook/{token}`) was written as an
unauthenticated endpoint — the 32-byte path-embedded `webhook_token`
generated by `secrets.token_urlsafe(32)` is the credential, and the
handler validates it against the row before doing anything. But
AuthMiddleware in app.py runs first and only knows about
AUTH_EXEMPT_EXACT (static path set) and AUTH_EXEMPT_PREFIXES (only
`/static`), so every external POST (curl, Zapier, n8n, Make,
Activepieces) got rejected before the route ever saw the request.
External callers can't supply a session cookie, which is precisely
why the per-task token exists.

Fix: add an AUTH_EXEMPT_PATTERNS list of compiled regexes for dynamic
public paths and route `^/api/tasks/[^/]+/webhook/[^/]+/?$` through
it. The route handler still enforces `ScheduledTask.webhook_token ==
token` and 404s on mismatch, so an attacker without the token gets a
404 (indistinguishable from a non-existent task), and a holder of the
token gets the documented "POST and a task fires" behaviour. The
sibling endpoint `/{task_id}/webhook-regenerate` is admin-gated and
deliberately does NOT match the pattern — it requires `_owner(request)`
and a session.

Tests: tests/test_webhook_trigger_auth_exempt.py extracts the regex
list out of app.py, applies it to a representative trigger path
(positive) and the four neighbouring task paths that must stay
authenticated (negative — `/api/tasks`, `/api/tasks/{id}`,
`/api/tasks/{id}/webhook-regenerate`, `/api/tasks/{id}/run`), and
pins the handler-side token check so a refactor of the route doesn't
quietly turn the endpoint into a truly anonymous one.

Closes #621.

tests/test_webhook_trigger_auth_exempt.py

@@ -0,0 +1,95 @@
+"""Pin the auth exemption for task webhook-trigger URLs.
+
+The task router exposes ``POST /api/tasks/{task_id}/webhook/{token}`` as a
+public webhook entrypoint — the path-embedded ``webhook_token`` is the
+credential, and the route handler in ``routes/task_routes.py`` validates
+it against the row and returns 404 on mismatch. The UI advertises the
+URL as "no auth needed" because external callers (Zapier, n8n, curl)
+can't supply a session cookie.
+
+Without an entry in ``AUTH_EXEMPT_PATTERNS`` ``AuthMiddleware`` rejected
+every POST with 401 before the token was ever checked (issue #621).
+This test re-reads the exemption logic out of ``app.py`` and confirms a
+representative webhook path is treated as exempt, while neighbouring
+non-public task paths are NOT.
+"""
+
+import os
+import re
+
+
+def _read_app_source() -> str:
+    app_path = os.path.join(
+        os.path.dirname(os.path.dirname(os.path.abspath(__file__))),
+        "app.py",
+    )
+    with open(app_path, encoding="utf-8") as fh:
+        return fh.read()
+
+
+def test_webhook_trigger_path_is_in_exempt_patterns():
+    """The dynamic webhook trigger path must match an AUTH_EXEMPT_PATTERNS
+    entry. Pull every regex literal compiled inside the block out of the
+    source and apply it directly — extraction has to tolerate nested
+    brackets inside each character class (e.g. ``[^/]+``)."""
+    src = _read_app_source()
+    # Find the start of the list, then walk character-by-character to the
+    # matching closing bracket. A regex would have to count brackets,
+    # which is more painful than just doing the count by hand.
+    start = src.find("AUTH_EXEMPT_PATTERNS")
+    assert start != -1, "AUTH_EXEMPT_PATTERNS not declared in app.py"
+    lb = src.find("[", start)
+    assert lb != -1
+    depth = 0
+    end = -1
+    for i in range(lb, len(src)):
+        ch = src[i]
+        if ch == "[":
+            depth += 1
+        elif ch == "]":
+            depth -= 1
+            if depth == 0:
+                end = i
+                break
+    assert end != -1, "could not find closing bracket for AUTH_EXEMPT_PATTERNS"
+    body = src[lb + 1 : end]
+    # Pull each compiled regex literal: _re.compile(r"...").
+    patterns = re.findall(r'_re\.compile\(\s*r"([^"]+)"\s*\)', body)
+    assert patterns, (
+        "expected at least one compiled regex in AUTH_EXEMPT_PATTERNS"
+    )
+    compiled = [re.compile(p) for p in patterns]
+
+    sample = "/api/tasks/abc123/webhook/" + "x" * 43
+    assert any(c.match(sample) for c in compiled), (
+        f"webhook trigger path {sample!r} must be auth-exempt - issue #621"
+    )
+
+    # Negative: routes that are NOT meant to be public must not match.
+    for not_public in (
+        "/api/tasks",
+        "/api/tasks/abc123",
+        "/api/tasks/abc123/webhook-regenerate",
+        "/api/tasks/abc123/run",
+    ):
+        assert not any(c.match(not_public) for c in compiled), (
+            f"{not_public!r} must NOT be auth-exempt"
+        )
+
+
+def test_webhook_trigger_handler_still_validates_token():
+    """The exemption is only safe because the route handler in
+    routes/task_routes.py still checks the token against the row and
+    returns 404 on mismatch. Pin that behaviour so a refactor of the
+    handler doesn't quietly make the endpoint truly anonymous. Read the
+    source directly — importing task_routes pulls in SQLAlchemy and
+    fails under the conftest stubs."""
+    routes_path = os.path.join(
+        os.path.dirname(os.path.dirname(os.path.abspath(__file__))),
+        "routes",
+        "task_routes.py",
+    )
+    with open(routes_path, encoding="utf-8") as fh:
+        src = fh.read()
+    assert "ScheduledTask.webhook_token == token" in src
+    assert '@router.post("/{task_id}/webhook/{token}")' in src