feat(db): trucks table, transport enum value, incidents.truck_id FK

- ALTER TYPE incident_type ADD VALUE 'transport' (isolated migration)
- trucks table with truck_no (unique), carrier, active, RLS mirroring sites
- incidents.truck_id UUID FK + index

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CPf5Rc8QPx6V8KLEEgfKEQ

supabase/migrations/20260713000004_trucks.sql

@@ -0,0 +1,14 @@
+CREATE TABLE trucks (
+  id         UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  truck_no   TEXT NOT NULL UNIQUE,
+  carrier    TEXT,
+  active     BOOLEAN NOT NULL DEFAULT true,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+ALTER TABLE incidents ADD COLUMN truck_id UUID REFERENCES trucks(id);
+CREATE INDEX incidents_truck_id_idx ON incidents(truck_id);
+
+ALTER TABLE trucks ENABLE ROW LEVEL SECURITY;
+CREATE POLICY "trucks_read"      ON trucks FOR SELECT USING (true);
+CREATE POLICY "trucks_admin_all" ON trucks FOR ALL    USING (auth_user_role() = 'admin');