Make new_suggestions and Send to Google Chat recoverable

MQ-167-01: a scheduled or manual batch now emits new_suggestions:{batchId}
in the transaction that first stores its Side quests, so the cards and
their notification commit together. A later round that fails, or a
failure recording the batch's end, no longer loses the event; a later
round's emit is a no-op by key; a notification that cannot be stored
fails that store, leaving no unannounced cards. Replenish batches still
never notify. finishBatch goes back to recording status only.

MQ-167-02: migration 0017 adds shopping_lists.chat_send_unconfirmed_at.
SendToChat records the send as begun before posting, clears it with
sent_to_chat_at once Google Chat accepts, and clears it alone when Chat
refuses. A post Chat does not answer, or one it accepts that cannot be
recorded, is ErrSendUnconfirmed (502 chat_unconfirmed, not retryable)
and leaves the marker, as does a crash mid-send; a repeat without resend
never reposts, and resend posts again. The Supply run screen re-reads
the list after a failed send, shows an unconfirmed send, offers no Retry
for it, and sends again only with resend.

Task: f0cd9d50-9c11-41be-a945-7bae9f5f28a4

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/db/models.go

@@ -276,15 +276,16 @@ type ShoppingItem struct {
 }
 
 type ShoppingList struct {
-	ID           string
-	WeekStart    string
-	PlanID       string
-	Title        sql.NullString
-	Status       string
-	SentToChatAt sql.NullString
-	FrozenAt     sql.NullString
-	CreatedAt    string
-	UpdatedAt    string
+	ID                    string
+	WeekStart             string
+	PlanID                string
+	Title                 sql.NullString
+	Status                string
+	SentToChatAt          sql.NullString
+	FrozenAt              sql.NullString
+	CreatedAt             string
+	UpdatedAt             string
+	ChatSendUnconfirmedAt sql.NullString
 }
 
 type Suggestion struct {