feat(finance): Phase 5 — Rules Engine

Add rules engine with CRUD API, condition/action evaluation, and apply-all endpoint.
UI: rule builder form with field/operator/value conditions, tag multi-select, apply button with result stats.

prisma/migrations/0005_rules/migration.sql

@@ -0,0 +1,12 @@
+CREATE TABLE IF NOT EXISTS rules (
+  id SERIAL PRIMARY KEY,
+  owner_id INTEGER NOT NULL REFERENCES participants(id),
+  name TEXT NOT NULL,
+  conditions JSONB NOT NULL DEFAULT '[]',
+  actions JSONB NOT NULL DEFAULT '{}',
+  enabled BOOLEAN NOT NULL DEFAULT true,
+  priority INTEGER NOT NULL DEFAULT 0,
+  created_at TIMESTAMPTZ DEFAULT NOW(),
+  updated_at TIMESTAMPTZ DEFAULT NOW()
+);
+CREATE INDEX IF NOT EXISTS idx_rules_owner ON rules(owner_id);

prisma/schema.prisma

@@ -65,3 +65,27 @@ model transaction_tags {
 
   @@id([transaction_id, tag_id])
 }
+
+model rules {
+  id         Int      @id @default(autoincrement())
+  owner_id   Int
+  name       String
+  conditions Json     @default("[]")
+  actions    Json     @default("{}")
+  enabled    Boolean  @default(true)
+  priority   Int      @default(0)
+  created_at DateTime @default(now())
+  updated_at DateTime @default(now()) @updatedAt
+}
+
+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])
+}