feat(user): add deterministic avatar color to AppUser

Adds color field assigned from an 8-colour palette keyed on the user's UUID
hash (Math.abs(id.hashCode()) % 8). Fires via @PrePersist/@PreUpdate/@PostLoad
so both new and existing users get the correct colour at runtime.

V47 migration adds the column and fixes the V46 REVOKE bug that hardcoded
role name 'app_user' instead of CURRENT_USER.

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

backend/src/main/resources/db/migration/V47__add_user_color.sql

@@ -0,0 +1,8 @@
+-- Add deterministic avatar color to app_users.
+-- Assigned at application layer (AppUser.java) from a fixed 8-colour palette.
+-- Also corrects V46's REVOKE which hardcoded 'app_user' instead of CURRENT_USER.
+
+ALTER TABLE app_users ADD COLUMN color VARCHAR(20) NOT NULL DEFAULT '';
+
+-- Fix V46 append-only enforcement for the actual application role.
+REVOKE UPDATE, DELETE ON audit_log FROM CURRENT_USER;