feat(tracing): add PostgreSQL tracing spans (#3678)

## Why
Expose PostgreSQL datastore latency within existing request traces so
slow logical database operations can be identified without recording
tenant data or query arguments.

## What
- Add client spans around logical PostgreSQL operations across the
database facade, search, audit, replica fencing, and command persistence
- Use a dedicated `buzz_datastore` target and `db.system.name =
"postgresql"` for filtering and backend classification
- Exclude health-check database calls and scrub raw identifiers and
errors from newly traced paths

## Risk Assessment
Medium — this instruments frequently used datastore paths and increases
trace volume when enabled, but does not change SQL execution or
datastore behavior. Existing OpenTelemetry filtering controls export.

## References
- Pre-push clippy and fast unit-test hooks passed

Generated with Amp

---------

Signed-off-by: David Grochowski <dgrochowski@squareup.com>
Co-authored-by: Amp <amp@ampcode.com>

crates/buzz-relay/Cargo.toml

@@ -19,6 +19,7 @@ path = "src/main.rs"
 buzz-core = { workspace = true }
 buzz-conformance = { workspace = true }
 buzz-db = { workspace = true }
+buzz-datastore-tracing = { workspace = true }
 buzz-auth = { workspace = true }
 buzz-pubsub = { workspace = true }
 buzz-audit = { workspace = true }

crates/buzz-relay/src/handlers/command_executor.rs

@@ -19,6 +19,7 @@ use uuid::Uuid;
 
 use buzz_core::kind::*;
 use buzz_core::tenant::{CommunityId, TenantContext};
+use buzz_datastore_tracing::datastore_span;
 use buzz_db::workflow::{ApprovalStatus, RunStatus};
 use buzz_db::DbError;
 use buzz_workflow::executor::TriggerContext;
@@ -97,6 +98,7 @@ enum PersistResult {
 /// persists without the event record. On retry, the event INSERT succeeds
 /// (no conflict), and the mutation re-executes — which is safe for idempotent
 /// operations (open_dm, hide_dm, update_approval, upsert_workflow).
+#[datastore_span(name = "persist_command_event", system = "postgresql")]
 async fn persist_command_event(
     state: &Arc<AppState>,
     tenant: &TenantContext,