feat(groups): add sinking_fund section and define four-group layout

Section enum gains sinking_fund with label "Sinking Funds". A new
groups module maps each section to one of Income, Committed, Savings,
Flexible and records the default open state per group. The existing
section column is a plain VARCHAR(32) with no CHECK, so no schema
migration is needed to accept the new value.

Refs #11

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/quartermaster/groups.py

@@ -0,0 +1,47 @@
+from __future__ import annotations
+
+import enum
+
+from quartermaster.models import Section
+
+
+class Group(str, enum.Enum):
+    income = "income"
+    committed = "committed"
+    savings = "savings"
+    flexible = "flexible"
+
+
+GROUP_LABELS: dict[Group, str] = {
+    Group.income: "Income",
+    Group.committed: "Committed",
+    Group.savings: "Savings",
+    Group.flexible: "Flexible",
+}
+
+
+GROUP_OF_SECTION: dict[Section, Group] = {
+    Section.income: Group.income,
+    Section.fixed_bill: Group.committed,
+    Section.debt_minimum: Group.committed,
+    Section.sinking_fund: Group.savings,
+    Section.food: Group.flexible,
+    Section.subscription: Group.flexible,
+    Section.other: Group.flexible,
+}
+
+
+GROUP_DEFAULT_OPEN: dict[Group, bool] = {
+    Group.income: True,
+    Group.committed: False,
+    Group.savings: False,
+    Group.flexible: True,
+}
+
+
+def sections_in_group(group: Group) -> list[Section]:
+    return [s for s in Section if GROUP_OF_SECTION[s] == group]
+
+
+def group_order() -> list[Group]:
+    return [Group.income, Group.committed, Group.savings, Group.flexible]

src/quartermaster/models.py

@@ -24,6 +24,7 @@ class Section(str, enum.Enum):
     food = "food"
     subscription = "subscription"
     other = "other"
+    sinking_fund = "sinking_fund"
 
 
 SECTION_LABELS: dict[Section, str] = {
@@ -33,6 +34,7 @@ SECTION_LABELS: dict[Section, str] = {
     Section.food: "Food and Essentials",
     Section.subscription: "Subscriptions",
     Section.other: "Other",
+    Section.sinking_fund: "Sinking Funds",
 }