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/tenant_isolation.rs

@@ -158,6 +158,70 @@ async fn tenant_db_name_sanitizes_unsafe_characters() {
     }
 }
 
+#[tokio::test]
+async fn admin_helpers_list_and_drop_tenant_dbs() {
+    let uri = std::env::var("TEST_MONGODB_URI")
+        .unwrap_or_else(|_| "mongodb://root:example@localhost:27017/?authSource=admin".into());
+    let prefix = format!("m72d_{}", short_id());
+    let pool = DatabasePool::connect(&uri, &prefix).await.expect("connect");
+
+    let acme = ctx("00000000-0000-0000-0000-00000000acme", "acme");
+    let globex = ctx("00000000-0000-0000-0000-0000globex000", "globex");
+
+    // Provision two tenants and write a doc into each so the databases
+    // actually materialize on the cluster (Mongo lazily creates DBs).
+    let acme_db = pool.for_tenant(&acme).await.expect("acme db");
+    let globex_db = pool.for_tenant(&globex).await.expect("globex db");
+    acme_db
+        .repositories()
+        .insert_one(fixture_repo("acme-app", "git@example.com:acme/app.git"))
+        .await
+        .expect("insert acme");
+    globex_db
+        .repositories()
+        .insert_one(fixture_repo("globex-app", "git@example.com:globex/app.git"))
+        .await
+        .expect("insert globex");
+
+    // list_tenant_db_names sees both, filtered by prefix
+    let names = pool.list_tenant_db_names().await.expect("list tenants");
+    let acme_name = pool.tenant_db_name(&acme.tenant_id);
+    let globex_name = pool.tenant_db_name(&globex.tenant_id);
+    assert!(
+        names.contains(&acme_name),
+        "expected {acme_name} in {names:?}"
+    );
+    assert!(
+        names.contains(&globex_name),
+        "expected {globex_name} in {names:?}"
+    );
+    for name in &names {
+        assert!(name.starts_with(&format!("{prefix}_")));
+    }
+
+    // drop_tenant removes acme's DB
+    pool.drop_tenant(&acme.tenant_id)
+        .await
+        .expect("drop acme tenant");
+    let after = pool
+        .list_tenant_db_names()
+        .await
+        .expect("list tenants after drop");
+    assert!(
+        !after.contains(&acme_name),
+        "acme should be gone after drop, got {after:?}"
+    );
+    assert!(
+        after.contains(&globex_name),
+        "globex should still be present, got {after:?}"
+    );
+
+    // Cleanup remaining
+    pool.drop_tenant(&globex.tenant_id)
+        .await
+        .expect("drop globex tenant");
+}
+
 #[tokio::test]
 async fn tenant_db_name_falls_back_to_hash_when_too_long() {
     let uri = std::env::var("TEST_MONGODB_URI")