feat(finance): Phase 6 — Budget & Analytics

Add monthly budgets per category with spend-vs-budget dashboard and 6-month trend table.
Includes upsert budget API, monthly analytics endpoint, inline budget editing, and route auth fixes.

prisma/migrations/0006_budgets/migration.sql

@@ -0,0 +1,11 @@
+CREATE TABLE IF NOT EXISTS budgets (
+  id SERIAL PRIMARY KEY,
+  owner_id INTEGER NOT NULL REFERENCES participants(id),
+  category TEXT NOT NULL,
+  month DATE NOT NULL,
+  amount_limit NUMERIC(10,2) NOT NULL,
+  created_at TIMESTAMPTZ DEFAULT NOW(),
+  updated_at TIMESTAMPTZ DEFAULT NOW(),
+  UNIQUE(owner_id, category, month)
+);
+CREATE INDEX IF NOT EXISTS idx_budgets_owner_month ON budgets(owner_id, month);

prisma/schema.prisma

@@ -65,3 +65,15 @@ model transaction_tags {
 
   @@id([transaction_id, tag_id])
 }
+
+model budgets {
+  id           Int      @id @default(autoincrement())
+  owner_id     Int
+  category     String
+  month        DateTime @db.Date
+  amount_limit Decimal  @db.Decimal(10, 2)
+  created_at   DateTime @default(now())
+  updated_at   DateTime @default(now()) @updatedAt
+
+  @@unique([owner_id, category, month])
+}