feat(05-01): migration SQL and Go model types for template system

- Create 002_templates.sql: item_tier enum, ALTER budget_items, templates and template_items tables with CHECK constraint
- Add ItemTier type with fixed/variable/one_off constants to models.go
- Add ItemTier field to BudgetItem struct
- Add Template, TemplateItem, TemplateDetail structs

backend/migrations/002_templates.sql

@@ -0,0 +1,27 @@
+CREATE TYPE item_tier AS ENUM ('fixed', 'variable', 'one_off');
+
+ALTER TABLE budget_items ADD COLUMN item_tier item_tier NOT NULL DEFAULT 'fixed';
+
+CREATE TABLE IF NOT EXISTS templates (
+    id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+    user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    name TEXT NOT NULL DEFAULT 'My Template',
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE UNIQUE INDEX idx_templates_user_id ON templates (user_id);
+
+CREATE TABLE IF NOT EXISTS template_items (
+    id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+    template_id UUID NOT NULL REFERENCES templates(id) ON DELETE CASCADE,
+    category_id UUID NOT NULL REFERENCES categories(id) ON DELETE RESTRICT,
+    item_tier item_tier NOT NULL,
+    budgeted_amount NUMERIC(12, 2),
+    sort_order INT NOT NULL DEFAULT 0,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    CONSTRAINT chk_template_items_no_one_off CHECK (item_tier IN ('fixed', 'variable'))
+);
+
+CREATE INDEX idx_template_items_template_id ON template_items (template_id);