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/test/http/hemhub-api.http

@@ -0,0 +1,55 @@
+### 1. Get access token for user "maria"
+POST http://localhost:8081/realms/hemhub/protocol/openid-connect/token
+Content-Type: application/x-www-form-urlencoded
+
+client_id=hemhub-public&
+grant_type=password&
+username=maria&
+password=Passw0rd
+
+> {%
+    client.global.set("token", response.body.access_token);
+    client.global.set("refresh_token", response.body.refresh_token);
+%}
+
+### 2. Decode token (optional, debugging)
+GET https://jwt.io/?access_token={{token}}
+
+### 3. Get current user info
+GET http://localhost:8080/me
+Authorization: Bearer {{token}}
+
+### 4. Create a household
+POST http://localhost:8080/api/v1/households
+Content-Type: application/json
+Authorization: Bearer {{token}}
+
+{
+  "name": "Familjen Andersson"
+}
+
+> {% client.global.set("householdId", JSON.parse(response.body).id); %}
+
+### 5. List my households
+GET http://localhost:8080/api/v1/households
+Authorization: Bearer {{token}}
+
+### 6. List household members
+GET http://localhost:8080/api/v1/households/{{householdId}}/members
+Authorization: Bearer {{token}}
+
+### 7. Add a member (requires OWNER role)
+POST http://localhost:8080/api/v1/households/{{householdId}}/members
+Content-Type: application/json
+Authorization: Bearer {{token}}
+
+{
+  "userSub": "ulf-sub",
+  "email": "ulf@example.com",
+  "displayName": "Ulf",
+  "role": "MEMBER"
+}
+
+### 8. List members again (should now include Ulf)
+GET http://localhost:8080/api/v1/households/{{householdId}}/members
+Authorization: Bearer {{token}}