feat(BE): approval_workflow, adjusment project_flocks, common, and migration

internal/database/migrations/20251015162158_create_approvals_table.up.sql

@@ -0,0 +1,12 @@
+CREATE TABLE approvals (
+    id BIGSERIAL PRIMARY KEY,
+    approvable_type VARCHAR(50) NOT NULL,
+    approvable_id BIGINT NOT NULL,
+    step SMALLINT NOT NULL,
+    status VARCHAR(20) NOT NULL,
+    notes TEXT,
+    created_at TIMESTAMPTZ DEFAULT NOW() NOT NULL,
+    action_by BIGINT REFERENCES users (id) ON DELETE SET NULL ON UPDATE CASCADE
+);
+
+CREATE INDEX approvals_approvable_lookup ON approvals (approvable_type, approvable_id);