fix(webui): validate externally influenced event_type at the timeline boundary (#637)

Review #526 found that event_type reached the serialized timeline payload
without crossing the redaction/validation boundary every other free-text
field on the same event crosses. A synthetic secret-shaped 40-hex canary
was redacted through message but survived verbatim through event_type on
the same control-plane record.

CTH heading path: CTH_TYPES is now the single authority for what a CTH
type may be. canonical_thread_handoff.is_known_cth_type() is that
authority, used by format_cth_body (write), assess_cth_comment (assess),
and now the read path too; parse_cth_comment reports membership as
cth_type_known and stays total. adapt_cth_comments serializes
handoff:<type> only for a declared type and otherwise emits the constant
handoff:unrecognized, so arbitrary, malformed, secret-shaped, or
whitespace-manipulated heading content never becomes an event_type.

Control-plane path: a stored event_type is treated as source data.
_safe_cp_event_type accepts only an ordinary identifier that is not a
bare secret-shaped hex run and that a redaction pass leaves unchanged;
anything else fails closed to the constant unsafe:redacted and marks the
event sensitive. The value is never emitted verbatim, never partially
sanitized, and never rewritten into a different valid-looking type.

Remaining serialized-field audit: event_key ids must be plain numeric
identifiers, the CTH adapter refuses a scope it cannot express, per-source
failure reasons are redacted (they can quote an authenticated fetch error),
and echoed scope/filter values are guarded so reflection is not a bypass.

Legitimate values are preserved: every declared CTH type, the real
producer types (assigned, lease_released, lease_adopted,
dependency_edge_state_change, allocation, pr.opened, lease.renew), and
the existing issue, PR, evidence, and full-SHA references.

Tests seed the canary independently through both event_type paths with a
benign message, so message redaction cannot be why they pass; each
inspects event_type directly, asserts the canary is absent from the
complete serialized payload, and asserts scan_for_secrets finds nothing.

tests/test_webui_timeline.py 64 passed, 15 subtests
pytest -k webui 362 passed, 285 subtests
pytest -k redact 79 passed
tests/test_canonical_thread_handoff.py tests/test_control_plane_db.py 29 passed

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

canonical_thread_handoff.py

@@ -46,6 +46,17 @@ _FIELD_RE = re.compile(
 )
 
 
+def is_known_cth_type(value: str | None) -> bool:
+    """True when *value* is a declared member of the :data:`CTH_TYPES` contract.
+
+    ``CTH_TYPES`` is the single authority for what a CTH type may be. The
+    heading a comment carries is free text, so a *read* path that turns a parsed
+    type into something durable — a serialized field, a routing decision — must
+    check membership here rather than trust the parse or keep a list of its own.
+    """
+    return (value or "").strip() in CTH_TYPES
+
+
 def format_cth_body(
     *,
     cth_type: str,
@@ -60,7 +71,7 @@ def format_cth_body(
 ) -> str:
     """Render a canonical CTH comment body."""
     normalized_type = (cth_type or "").strip()
-    if normalized_type not in CTH_TYPES:
+    if not is_known_cth_type(normalized_type):
         raise ValueError(
             f"unknown CTH type '{cth_type}'; expected one of {sorted(CTH_TYPES)}"
         )
@@ -101,6 +112,12 @@ def parse_cth_comment(body: str) -> dict[str, Any] | None:
         fields[key] = match.group(2).strip()
     return {
         "cth_type": cth_type,
+        # The heading capture is unconstrained free text, so the parse states
+        # whether it satisfies the CTH_TYPES contract instead of leaving every
+        # reader to decide (or forget). Parsing stays total — an unknown type is
+        # still parsed and reported, never raised on — but a reader that turns
+        # the type into a durable value can now tell the two apart.
+        "cth_type_known": is_known_cth_type(cth_type),
         "fields": fields,
         "raw_body": text,
     }
@@ -119,7 +136,7 @@ def assess_cth_comment(body: str) -> dict[str, Any]:
         }
 
     cth_type = parsed.get("cth_type") or ""
-    if cth_type not in CTH_TYPES:
+    if not is_known_cth_type(cth_type):
         reasons.append(
             f"unknown CTH type '{cth_type}'; expected one of {sorted(CTH_TYPES)}"
         )