feat(m7.2-B): migrate API handlers to per-tenant database pool

Builds on PR M7.2-A. Every HTTP handler in compliance-agent/src/api/
now takes a TenantCtx extractor and pulls a tenant-scoped Database
from agent.db_pool.for_tenant(&ctx). The query bodies are unchanged —
`db.findings().find(doc! {...})` reads from the tenant's own physical
database, so the filter doc cannot leak data across tenants because
the wrong tenant's data is literally on a different db handle.

Changes
- New `dto::tenant_db(&agent, &tenant) -> Result<Database, StatusCode>`
  helper. Every migrated handler calls it at the top of the body
  instead of `let db = &agent.db;`. 500 on the rare pool failure;
  4xx auth failures are already handled by the M7.1 status gate.
- New `api::server::inject_dev_tenant` middleware mounted only when
  Keycloak is NOT configured. Synthesizes a TenantContext with
  tenant_id = $DEV_TENANT_ID (default `dev`) so `cargo run` against
  a bare Mongo + no KC still serves the API. Logged loudly as
  "DO NOT use in any environment with real customer data".
- Test harness: TestServer mounts inject_dev_tenant so existing E2E
  tests reach handlers; cleanup() now drops every <db_name>_*
  per-tenant database, not just the legacy <db_name>.

Files migrated (handler count, all pass `cargo build`):
- chat.rs (3) — also rewires RagPipeline + EmbeddingStore to the
  tenant DB's inner() so vector search is per-tenant
- dast.rs (5)
- findings.rs (5)
- graph.rs (7) — also rewires GraphStore inside trigger_build's
  spawn to the tenant DB
- health.rs (1) — stats_overview migrated; public /health stays
  un-scoped
- issues.rs (1)
- notifications.rs (5)
- pentest_handlers/session.rs (12) — both wizard + legacy paths,
  plus pause/resume/stop/get_attack_chain/get_messages/
  get_session_findings/lookup_repo. PentestOrchestrator now gets
  the tenant DB clone in its spawn.
- pentest_handlers/export.rs (1) — fans out across sessions,
  attack_chain_nodes, dast_findings, findings, sbom_entries,
  graph_nodes from a single tenant_db acquisition
- pentest_handlers/stats.rs (1)
- pentest_handlers/stream.rs (1) — SSE handler verifies session
  via the tenant DB before subscribing
- repos.rs (6)
- sbom.rs (5)
- scans.rs (1)

help_chat.rs has no DB queries and was skipped.

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 — 5 pass
  (driver-level isolation still holds post-handler migration)
- cargo test -p compliance-agent --test tenant_status_middleware
  — 6 pass

What's not yet migrated (PR-C / PR-D)
- scheduler.rs (6 sites), pipeline/orchestrator.rs (14),
  pentest/orchestrator.rs (13), webhooks (gitea/github/gitlab),
  trackers/jira.rs, pipeline/dedup.rs etc. — background paths
  without a JWT-derived tenant context.
- agent.db is still in the ComplianceAgent struct as a transitional
  handle for those paths. PR-D removes it once PR-C migrates the
  background paths.

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

compliance-agent/src/api/handlers/pentest_handlers/stats.rs

@@ -6,10 +6,11 @@ use axum::Json;
 use mongodb::bson::doc;
 
 use compliance_core::models::pentest::*;
+use compliance_core::tenant_ctx::TenantCtx;
 
 use crate::agent::ComplianceAgent;
 
-use super::super::dto::{collect_cursor_async, ApiResponse};
+use super::super::dto::{collect_cursor_async, tenant_db, ApiResponse};
 
 type AgentExt = Extension<Arc<ComplianceAgent>>;
 
@@ -17,8 +18,10 @@ type AgentExt = Extension<Arc<ComplianceAgent>>;
 #[tracing::instrument(skip_all)]
 pub async fn pentest_stats(
     Extension(agent): AgentExt,
+    tenant: TenantCtx,
 ) -> Result<Json<ApiResponse<PentestStats>>, StatusCode> {
-    let db = &agent.db;
+    let db = tenant_db(&agent, &tenant).await?;
+    let db = &db;
 
     let running_sessions = db
         .pentest_sessions()