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/export.rs

@@ -13,10 +13,11 @@ use compliance_core::models::dast::DastFinding;
 use compliance_core::models::finding::Finding;
 use compliance_core::models::pentest::*;
 use compliance_core::models::sbom::SbomEntry;
+use compliance_core::tenant_ctx::TenantCtx;
 
 use crate::agent::ComplianceAgent;
 
-use super::super::dto::collect_cursor_async;
+use super::super::dto::{collect_cursor_async, tenant_db};
 
 type AgentExt = Extension<Arc<ComplianceAgent>>;
 
@@ -35,11 +36,15 @@ pub struct ExportBody {
 #[tracing::instrument(skip_all, fields(session_id = %id))]
 pub async fn export_session_report(
     Extension(agent): AgentExt,
+    tenant: TenantCtx,
     Path(id): Path<String>,
     Json(body): Json<ExportBody>,
 ) -> Result<axum::response::Response, (StatusCode, String)> {
     let oid = mongodb::bson::oid::ObjectId::parse_str(&id)
         .map_err(|_| (StatusCode::BAD_REQUEST, "Invalid session ID".to_string()))?;
+    let db = tenant_db(&agent, &tenant)
+        .await
+        .map_err(|s| (s, "failed to acquire tenant database".to_string()))?;
 
     if body.password.len() < 8 {
         return Err((
@@ -49,8 +54,7 @@ pub async fn export_session_report(
     }
 
     // Fetch session
-    let session = agent
-        .db
+    let session = db
         .pentest_sessions()
         .find_one(doc! { "_id": oid })
         .await
@@ -64,9 +68,7 @@ pub async fn export_session_report(
 
     // Resolve target name
     let target = if let Ok(tid) = mongodb::bson::oid::ObjectId::parse_str(&session.target_id) {
-        agent
-            .db
-            .dast_targets()
+        db.dast_targets()
             .find_one(doc! { "_id": tid })
             .await
             .ok()
@@ -84,8 +86,7 @@ pub async fn export_session_report(
         .unwrap_or_default();
 
     // Fetch attack chain nodes
-    let nodes: Vec<AttackChainNode> = match agent
-        .db
+    let nodes: Vec<AttackChainNode> = match db
         .attack_chain_nodes()
         .find(doc! { "session_id": &id })
         .sort(doc! { "started_at": 1 })
@@ -96,8 +97,7 @@ pub async fn export_session_report(
     };
 
     // Fetch DAST findings for this session, then deduplicate
-    let raw_findings: Vec<DastFinding> = match agent
-        .db
+    let raw_findings: Vec<DastFinding> = match db
         .dast_findings()
         .find(doc! { "session_id": &id })
         .sort(doc! { "severity": -1, "created_at": -1 })
@@ -122,8 +122,7 @@ pub async fn export_session_report(
         .or_else(|| target.as_ref().and_then(|t| t.repo_id.clone()));
 
     let (sast_findings, sbom_entries, code_context) = if let Some(ref rid) = repo_id {
-        let sast: Vec<Finding> = match agent
-            .db
+        let sast: Vec<Finding> = match db
             .findings()
             .find(doc! {
                 "repo_id": rid,
@@ -143,8 +142,7 @@ pub async fn export_session_report(
             Err(_) => Vec::new(),
         };
 
-        let sbom: Vec<SbomEntry> = match agent
-            .db
+        let sbom: Vec<SbomEntry> = match db
             .sbom_entries()
             .find(doc! {
                 "repo_id": rid,
@@ -164,8 +162,7 @@ pub async fn export_session_report(
         };
 
         // Build code context from graph nodes
-        let code_ctx: Vec<CodeContextHint> = match agent
-            .db
+        let code_ctx: Vec<CodeContextHint> = match db
             .graph_nodes()
             .find(doc! { "repo_id": rid, "is_entry_point": true })
             .limit(50)