Add search-users-by-email endpoint

[evanpleasic-crypto]

Summary

• Adds a /user/search endpoint (SearchUserAction) that looks up users by a partial email match
• Backed by a parameterized JPQL named query (findUsersByEmail) rather than string-concatenated SQL, avoiding SQL injection in the lookup
• Wires the new repository/service methods (searchUsersByEmail) through the existing JPA-based UserRepository/UserService layers, following the same pattern as the existing todo-search feature
• Adds a search box to the sidebar and a results view (user/search.jsp)

Test plan

• Deploy the app and log in
• Use the new "Search users" box in the sidebar to search by a partial email
• Confirm matching users are listed and no results shows the "no user" message
• Confirm input containing SQL metacharacters (e.g. ' OR '1'='1) is treated as a literal search term, not SQL syntax

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