fix(telemetry): simplify Sentry configuration metadata

[NathanFlurry]

• Replace full configuration dumps in Sentry tags with concise database, pubsub, and cache backend labels.
• Remove topology metadata from the Sentry scope while preserving configuration loading and the telemetry opt-out.
• Validate with cargo check -p rivet-telemetry.

[railway-app]

🚅 Deployed to the rivet-pr-5854 environment in rivet-frontend

8 services not affected by this PR

• frontend-cloud
• frontend-inspector
• website
• ladle
• sentry-tunnel
• kitchen-sink
• mcp-hub
• frontend-cloud-ee

[claude]

Review: fix(telemetry): simplify Sentry configuration metadata

Summary: This removes a real leak. Serializing the full Database, PubSub, and Cache configs into Sentry tags could send credentials such as Postgres or NATS URLs with passwords. Tagging only fixed labels fixes that. I found no blocking issues.

Correctness

• The enum matches are exhaustive, with no _ => arm. This follows CLAUDE.md, and a new Database, PubSub, or CacheDriver variant will fail to compile instead of silently going untagged.
• The import paths resolve. rivet_config::config re-exports Database, PubSub, and CacheDriver, and Cache::driver() and enabled exist.
• Dropping serde_json from Cargo.toml and Cargo.lock is consistent, since nothing else in the crate uses it.
• The tag values are bounded and low-cardinality, which suits Sentry tags.

Minor notes

• The topology tag is removed entirely. That fits the description, but anyone filtering Sentry by topology will lose that dimension.
• The comment on the allowlist is clear. If you reword it, use periods instead of any dashes, per the CLAUDE.md comment rules.
• Tests: there are none, and a pure tag mapping makes that acceptable. If you want one, a test under tests/ could check that the tags never contain URL-like content. I did not run cargo check -p rivet-telemetry myself.

LGTM.