Remove unnecessary timestamps from EC identity KV entries

[ChristianPavilonis]

Summary

Remove all timestamp fields from the ec_identity_store KV schema except:

• KvEntry.created
• KvConsent.updated (consent.updated in JSON)

Current timestamp fields

The KvEntry body currently stores several Unix timestamp fields:

• created — keep
• last_seen — remove
• consent.updated — keep
• ids.*.synced — remove
• pub_properties.seen_domains.*.first — remove
• pub_properties.seen_domains.*.last — remove
• network.cluster_checked — remove

Fastly KV TTLs are applied separately and should remain outside the serialized body.

Motivation

Minimize retained temporal data in the EC identity graph. The KV entry should retain only the timestamps necessary for record creation and consent state tracking.

Acceptance criteria

• Update crates/trusted-server-core/src/ec/kv_types.rs schema types to remove the non-allowed timestamp fields.
• Update all constructors and mutators that currently populate or update removed timestamp fields.
• Update serialization/deserialization tests and fixtures.
• Ensure legacy records containing now-removed timestamp fields deserialize safely, if applicable.
• Keep created and consent.updated unchanged.
• Run the relevant Rust test suite and formatting/lint checks.