fix(invite): address review cycle 2 feedback

- Narrow isTrustedProxy to RFC 1918 172.16-31.x.x (was 172.x.x.x)
- Add @Valid/@NotBlank/@Email to RegisterRequest and @Valid to AuthController
- Add FK constraint on invite_token_group_ids.group_id → user_groups(id)
- Add back-to-login link and <main> landmark to register error state
- Add component test suite for register/+page.svelte (11 tests)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/resources/db/migration/V45__add_invite_tokens.sql

@@ -17,6 +17,6 @@ CREATE INDEX idx_invite_tokens_code ON invite_tokens(code);
 
 CREATE TABLE invite_token_group_ids (
     invite_token_id UUID NOT NULL REFERENCES invite_tokens(id),
-    group_id        UUID NOT NULL,
+    group_id        UUID NOT NULL REFERENCES user_groups(id),
     PRIMARY KEY (invite_token_id, group_id)
 );