feat: allow multi tags for budgets

internal/db/migrations/00013_budget_tags_table.sql

@@ -0,0 +1,38 @@
+-- +goose Up
+-- Create budget_tags join table for many-to-many relationship
+CREATE TABLE budget_tags (
+    budget_id TEXT NOT NULL,
+    tag_id TEXT NOT NULL,
+    PRIMARY KEY (budget_id, tag_id),
+    FOREIGN KEY (budget_id) REFERENCES budgets(id) ON DELETE CASCADE,
+    FOREIGN KEY (tag_id) REFERENCES tags(id) ON DELETE CASCADE
+);
+
+CREATE INDEX idx_budget_tags_tag_id ON budget_tags(tag_id);
+
+-- Migrate existing tag_id data to the join table
+INSERT INTO budget_tags (budget_id, tag_id)
+SELECT id, tag_id FROM budgets WHERE tag_id IS NOT NULL;
+
+-- Drop the unique constraint and FK that reference tag_id, then drop the column
+ALTER TABLE budgets DROP CONSTRAINT budgets_space_id_tag_id_period_key;
+ALTER TABLE budgets DROP CONSTRAINT budgets_tag_id_fkey;
+ALTER TABLE budgets DROP COLUMN tag_id;
+
+-- +goose Down
+-- Add tag_id column back
+ALTER TABLE budgets ADD COLUMN tag_id TEXT;
+
+-- Copy first tag back from budget_tags
+UPDATE budgets SET tag_id = (
+    SELECT tag_id FROM budget_tags WHERE budget_tags.budget_id = budgets.id LIMIT 1
+);
+
+-- Re-add FK and unique constraint
+ALTER TABLE budgets ADD CONSTRAINT budgets_tag_id_fkey
+    FOREIGN KEY (tag_id) REFERENCES tags(id) ON DELETE CASCADE;
+ALTER TABLE budgets ADD CONSTRAINT budgets_space_id_tag_id_period_key
+    UNIQUE (space_id, tag_id, period);
+
+DROP INDEX IF EXISTS idx_budget_tags_tag_id;
+DROP TABLE IF EXISTS budget_tags;