Add household and membership domain with role-based APIs

Introduced `Household` and `HouseholdMember` entities, services, and repositories, enabling role-based access (`OWNER` and `MEMBER`). Added REST APIs for household creation, member management, and listing. Configured Flyway migrations, updated tests, and included IntelliJ HTTP client script for API testing. Updated `README.md` with feature details, usage instructions, and architecture overview.

src/main/resources/db/migration/V2__household.sql

@@ -0,0 +1,26 @@
+-- src/main/resources/db/migration/V2__households.sql
+
+-- Households & Members (H2/PG compatible)
+CREATE TABLE households (
+                            id UUID PRIMARY KEY,
+                            name VARCHAR(120) NOT NULL,
+                            created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE TABLE household_members (
+                                   id UUID PRIMARY KEY,
+                                   household_id UUID NOT NULL REFERENCES households(id) ON DELETE CASCADE,
+                                   user_sub VARCHAR(64) NOT NULL,
+                                   email VARCHAR(255),
+                                   display_name VARCHAR(120),
+                                   role VARCHAR(16) NOT NULL,
+                                   created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+                                   UNIQUE (household_id, user_sub)
+);
+
+-- Optional: enforce role values (H2 accepts the CHECK too)
+ALTER TABLE household_members
+    ADD CONSTRAINT chk_household_role CHECK (role IN ('OWNER','MEMBER'));
+
+CREATE INDEX idx_household_members_user_sub ON household_members(user_sub);
+CREATE INDEX idx_household_members_household ON household_members(household_id);