feat(m7.2-D): drop transitional agent.db, add admin helpers

Final slice of M7.2. Removes the transitional single-database handle
that M7.2-A introduced alongside the pool, so the compliance-agent
now has a single source of truth for storage: every code path obtains
a tenant-scoped Database from `agent.db_pool.for_tenant_id(...)` or
`for_tenant(&ctx)`. There is no shared "default" database anywhere.

Changes
- ComplianceAgent: `db: Database` field removed. ComplianceAgent::new
  now takes only `(config, db_pool)`. Verified by an earlier grep
  during M7.2-C that no remaining call site reads `agent.db`.
- main.rs: stops constructing the legacy Database. Only the pool is
  built at startup.
- TestServer: same — drops Database::connect/ensure_indexes, builds
  only the pool. cleanup() now drops every `<db_name>_*` per-tenant
  database (no longer touches a bare `<db_name>`).
- DatabasePool::list_tenant_db_names() — lists Mongo databases
  matching the pool's prefix. For admin endpoints + scheduler tenant
  enumeration in a future M7.3 (this PR keeps SCHEDULER_TENANT_IDS
  env config — registry integration is a separate concern).
- DatabasePool::drop_tenant(&str) — idempotent tenant offboarding.
  Drops the per-tenant database and evicts the in-memory `ensured`
  marker so a later re-provision re-runs ensure_indexes.

Test plan
- cargo fmt --all clean
- cargo clippy --workspace --exclude compliance-dashboard
  -- -D warnings clean
- cargo test -p compliance-core --lib — 7 pass
- cargo test -p compliance-agent --lib — 228 pass
- cargo test -p compliance-agent --test tenant_isolation — 6 pass
  including new `admin_helpers_list_and_drop_tenant_dbs`
- cargo test -p compliance-agent --test tenant_status_middleware
  — 6 pass

M7.2 closeout state after this lands
- M7.1 (auth + status) — done
- M7.2-A (pool) — done
- M7.2-B (handlers) — done
- M7.2-C (background paths) — done
- M7.2-D (legacy db removed, admin helpers) — done (this PR)
- Future M7.3: scheduler pulls tenants from tenant-registry instead
  of SCHEDULER_TENANT_IDS env; cross-tenant admin HTTP endpoints
  built on list_tenant_db_names / drop_tenant.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

compliance-agent/tests/common/mod.rs

@@ -7,7 +7,7 @@ use std::sync::Arc;
 
 use compliance_agent::agent::ComplianceAgent;
 use compliance_agent::api;
-use compliance_agent::database::{Database, DatabasePool};
+use compliance_agent::database::DatabasePool;
 use compliance_core::AgentConfig;
 use secrecy::SecretString;
 
@@ -28,11 +28,6 @@ impl TestServer {
         // Unique database name per test run to avoid collisions
         let db_name = format!("test_{}", uuid::Uuid::new_v4().simple());
 
-        let db = Database::connect(&mongodb_uri, &db_name)
-            .await
-            .expect("Failed to connect to MongoDB — is it running?");
-        db.ensure_indexes().await.expect("Failed to create indexes");
-
         let db_pool = DatabasePool::connect(&mongodb_uri, &db_name)
             .await
             .expect("Failed to build DatabasePool");
@@ -73,7 +68,7 @@ impl TestServer {
             pentest_imap_password: None,
         };
 
-        let agent = ComplianceAgent::new(config, db, db_pool);
+        let agent = ComplianceAgent::new(config, db_pool);
 
         // Build the router with the agent extension. After M7.2-B every
         // handler takes a TenantCtx extractor; without KC in the test
@@ -164,12 +159,11 @@ impl TestServer {
         &self.db_name
     }
 
-    /// Drop the test database on cleanup. Post-M7.2-B the actual data
-    /// lives in `<db_name>_<tenant>` per-tenant databases; list those
-    /// off the cluster and drop them too.
+    /// Drop every per-tenant database belonging to this test run.
+    /// Post-M7.2-D the agent never opens a `db_name` directly —
+    /// data lives only in `<db_name>_<tenant>` per-tenant databases.
     pub async fn cleanup(&self) {
         if let Ok(client) = mongodb::Client::with_uri_str(&self.mongodb_uri).await {
-            client.database(&self.db_name).drop().await.ok();
             if let Ok(names) = client.list_database_names().await {
                 let prefix = format!("{}_", self.db_name);
                 for name in names {