Fix fetch-classify INIT crash and thread-state writes (#41)

fetch-classify has been down since 2026-05-05 (100% error rate, every
5-min poll failing silently at INIT). Three stacked failures:

- typing_extensions was not resolved into the bundle by PythonFunction
  bundling, so every import died with Runtime.ImportModuleError. Pin it
  explicitly in src/requirements.txt.
- DynamoDB UpdateExpressions used the reserved keyword ttl unaliased,
  failing every save_thread_state/update_task_status call once imports
  were fixed. Alias as #ttl via ExpressionAttributeNames.
- (Out-of-band) classification model claude-haiku-4-5 was never enabled
  in Bedrock model access; agreement accepted account-side 2026-06-03.

Verified live: manual invoke returns processed=1 errors=0.

src/requirements.txt

@@ -2,3 +2,6 @@ google-api-python-client>=2.197.0
 google-auth>=2.53.0
 python-ulid>=3.1.0
 requests>=2.34.2
+# Transitive dep not reliably resolved during bundling; absence breaks all
+# imports at INIT (Runtime.ImportModuleError, fetch-classify down 05-05→06-03)
+typing_extensions>=4.12

src/shared/dynamo.py

@@ -71,9 +71,10 @@ def save_thread_state(thread_id, message, adam_email):
                 "last_message_date = :now, "
                 "subject = :subject, "
                 "adam_last_reply_date = :now, "
-                "ttl = :ttl "
+                "#ttl = :ttl "
                 "REMOVE unanswered_since"
             ),
+            ExpressionAttributeNames={"#ttl": "ttl"},
             ExpressionAttributeValues={
                 ":from_email": message["from_email"],
                 ":now": now_iso,
@@ -89,8 +90,9 @@ def save_thread_state(thread_id, message, adam_email):
                 "last_message_date = :now, "
                 "subject = :subject, "
                 "unanswered_since = if_not_exists(unanswered_since, :now), "
-                "ttl = :ttl"
+                "#ttl = :ttl"
             ),
+            ExpressionAttributeNames={"#ttl": "ttl"},
             ExpressionAttributeValues={
                 ":from_email": message["from_email"],
                 ":now": now_iso,
@@ -252,7 +254,7 @@ def get_tasks(status_filter=None):
 
 
 def update_task_status(task_id, status):
-    update_expr = "SET #s = :status, ttl = :ttl"
+    update_expr = "SET #s = :status, #ttl = :ttl"
     expr_values = {":status": status, ":ttl": _ttl_90_days()}
     if status == "completed":
         update_expr += ", completed_at = :now"
@@ -261,7 +263,7 @@ def update_task_status(task_id, status):
     _get_table().update_item(
         Key={"pk": f"TASK#{task_id}", "sk": "TASK"},
         UpdateExpression=update_expr,
-        ExpressionAttributeNames={"#s": "status"},
+        ExpressionAttributeNames={"#s": "status", "#ttl": "ttl"},
         ExpressionAttributeValues=expr_values,
     )