feat: support unassigned purchases and allocation status

Allow month-only purchases to start finance plans. Keep unbudgeted posts distinct from explicit zero rules in overspend reporting.

docs/PRODUCT.md

@@ -59,10 +59,15 @@ Confirmed and in place:
   posts alone hold envelope balances.
 - Posts (envelopes) with monthly allocation, created and archived by the owner.
   Posts are archived, never deleted, because purchases reference them.
+- A post with no applicable allocation rule still records spending and shows its
+  balance, but is not labelled overspent. An explicit zero allocation is a
+  budget rule and can be overspent.
 - Allocation is either a fixed amount or a percentage of that month's income.
 - A purchase can be split across several posts, by percentage or by fixed
   amounts, and can also be spread across future months as a payment plan. The
   two compose.
+- A purchase may have no post. It remains visible in its month and counts in
+  Spent, while leaving envelope balances unchanged.
 - Per-month income, entered directly, varying month to month.
 - Multi-currency (DKK/USD/EUR) against a user-maintained exchange-rate table,
   with an optional on-demand fetch that fails soft and is never required.