feat(sales-summaries): log a summary's imbalance and its suspect terms

Adds d-ss/imbalance alongside balanced?, and logs it from sales-summaries-v2
together with the categories a balancing investigation keeps returning to.

balanced? answers yes or no; when the answer is no there is currently nothing
recording by how much or in which direction, so an out-of-balance day is only
visible by opening the grid. This turns it into something queryable, and gives
the calculation changes that follow a before-and-after signal rather than an
opinion. No behaviour change.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/clj/auto_ap/datomic/sales_summaries.clj

@@ -36,6 +36,13 @@
 (defn balanced? [items]
   (dollars= (total-debits items) (total-credits items)))
 
+(defn imbalance
+  "Signed debits minus credits. `balanced?` answers yes or no; this says by how much and in
+  which direction, so a day that does not balance can be logged and queried rather than only
+  rendered red. Positive means the tender side exceeds what revenue accounts for."
+  [items]
+  (- (total-debits items) (total-credits items)))
+
 (defn accepted?
   "True once a summary is finished: every line is mapped to an account and debits equal
   credits. This is the same condition the sales summaries grid renders as \"Balanced\", and