Add per-listener connection limits

Configurable maximum concurrent connections per listener. When the
limit is reached, new connections are closed immediately after accept.
0 means unlimited (default, preserving existing behavior).

Config: Listener gains max_connections field, validated non-negative.

DB: Migration 3 adds listeners.max_connections column.
UpdateListenerMaxConns method for runtime changes via gRPC.
CreateListener updated to persist max_connections on seed.

Server: ListenerState/ListenerData gain MaxConnections. Limit checked
in serve() after Accept but before handleConn — if ActiveConnections
>= MaxConnections, connection is closed and the accept loop continues.
SetMaxConnections method for runtime updates.

Proto: SetListenerMaxConnections RPC added. ListenerStatus gains
max_connections field. Generated code regenerated.

gRPC server: SetListenerMaxConnections implements write-through
(DB first, then in-memory update). GetStatus includes max_connections.

Client: SetListenerMaxConnections method, MaxConnections in
ListenerStatus.

Tests: DB CRUD and UpdateListenerMaxConns, server connection limit
enforcement (accept 2, reject 3rd, close one, accept again), gRPC
SetListenerMaxConnections round-trip with DB persistence, not-found
error handling.

Also updates PROJECT_PLAN.md with phases 6-8 and PROGRESS.md with
tracking for the new features.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

PROJECT_PLAN.md

@@ -329,3 +329,48 @@ Test that htmx-based web UIs work through the L7 proxy:
 - Verify ARCHITECTURE.md matches final implementation.
 - Update CLAUDE.md if any package structure or rules changed.
 - Update Makefile if new build targets are needed.
+
+---
+
+## Phase 6: Per-Listener Connection Limits
+
+Add configurable maximum concurrent connection limits per listener.
+
+### 6.1 Config: `MaxConnections int64` on `Listener` (0 = unlimited)
+### 6.2 DB: migration 3 adds `listeners.max_connections`, CRUD updates
+### 6.3 Server: enforce limit in `serve()` after Accept, before handleConn
+### 6.4 Proto/gRPC: `SetListenerMaxConnections` RPC, `max_connections` in `ListenerStatus`
+### 6.5 Client/CLI: `SetListenerMaxConnections` method, status display
+### 6.6 Tests: DB CRUD, server limit enforcement, gRPC round-trip
+
+---
+
+## Phase 7: L7 Policies
+
+Per-route HTTP blocking rules for L7 routes: user-agent blocking
+(substring match) and required header enforcement.
+
+### 7.1 Config: `L7Policy` struct (`type` + `value`), `L7Policies` on Route
+### 7.2 DB: migration 4 creates `l7_policies` table, new `l7policies.go` CRUD
+### 7.3 L7 middleware: `PolicyMiddleware` in `internal/l7/policy.go`
+### 7.4 Server/L7 integration: thread policies from RouteInfo to RouteConfig
+### 7.5 Proto/gRPC: `L7Policy` message, `ListL7Policies`/`AddL7Policy`/`RemoveL7Policy` RPCs
+### 7.6 Client/CLI: policy methods, `mcproxyctl policies` subcommand
+### 7.7 Startup: load L7 policies per route in `loadListenersFromDB`
+### 7.8 Tests: middleware unit tests, DB CRUD + cascade, gRPC round-trip, e2e
+
+---
+
+## Phase 8: Prometheus Metrics
+
+Instrument the proxy with Prometheus-compatible metrics exposed via a
+separate HTTP endpoint.
+
+### 8.1 Dependency: add `prometheus/client_golang`
+### 8.2 Config: `Metrics` section (`addr`, `path`)
+### 8.3 Package: `internal/metrics/` with metric definitions and HTTP server
+### 8.4 Instrumentation: connections, firewall blocks, dial latency, bytes, HTTP status codes, policy blocks
+### 8.5 Firewall: add `BlockedWithReason()` method
+### 8.6 L7: status recording wrapper on ResponseWriter
+### 8.7 Startup: conditionally start metrics server
+### 8.8 Tests: metric sanity, server endpoint, `BlockedWithReason`