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);