fix(auth): map unique email violation to Conflict on register (#55)

[Simultech369]

Summary

Resolves #55 by catching PostgreSQL unique constraint violations on INSERT INTO users during registration and mapping them to HTTP 409 Conflict (AppError::Conflict("Email".into())), matching the semantic behavior of the preliminary existence check and eliminating spurious 500 INTERNAL_SERVER_ERROR responses under concurrent registration races.

Changes

1. Shared Unique Violation Detection (src/error.rs):
   • Promoted is_unique_violation(error: &sqlx::Error) -> bool from src/services/batch.rs to src/error.rs.
   • Reused across both batch.rs and routes/auth.rs.
2. Registration Handler (src/routes/auth.rs):
   • Retained preliminary SELECT EXISTS check as an optimization to avoid paying unnecessary bcrypt hashing computation costs for obvious duplicates.
   • Wrapped INSERT INTO users query execution with .map_err(...):
     • If is_unique_violation(&e) is true, maps to AppError::Conflict("Email".into()) (HTTP 409).
     • Otherwise forwards original AppError::Database(e) (HTTP 500).
3. Unit Tests (src/error.rs):
   • test_conflict_status_and_code: Asserts AppError::Conflict maps to StatusCode::CONFLICT and machine-readable code "CONFLICT".
   • test_is_unique_violation_non_db_error: Asserts non-unique errors (e.g. RowNotFound) evaluate to false.
   • All 40 unit tests pass hermetically in cargo test.

Closes #55

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