Add L7 policies for user-agent blocking and required headers

Per-route HTTP-level blocking policies for L7 routes. Two rule types:
block_user_agent (substring match against User-Agent, returns 403)
and require_header (named header must be present, returns 403).

Config: L7Policy struct with type/value fields, added as L7Policies
slice on Route. Validated in config (type enum, non-empty value,
warning if set on L4 routes).

DB: Migration 4 creates l7_policies table with route_id FK (cascade
delete), type CHECK constraint, UNIQUE(route_id, type, value). New
l7policies.go with ListL7Policies, CreateL7Policy, DeleteL7Policy,
GetRouteID. Seed updated to persist policies from config.

L7 middleware: PolicyMiddleware in internal/l7/policy.go evaluates
rules in order, returns 403 on first match, no-op if empty. Composed
into the handler chain between context injection and reverse proxy.

Server: L7PolicyRule type on RouteInfo with AddL7Policy/RemoveL7Policy
mutation methods on ListenerState. handleL7 threads policies into
l7.RouteConfig. Startup loads policies per L7 route from DB.

Proto: L7Policy message, repeated l7_policies on Route. Three new
RPCs: ListL7Policies, AddL7Policy, RemoveL7Policy. All follow the
write-through pattern.

Client: L7Policy type, ListL7Policies/AddL7Policy/RemoveL7Policy
methods. CLI: mcproxyctl policies list/add/remove subcommands.

Tests: 6 PolicyMiddleware unit tests (no policies, UA match/no-match,
header present/absent, multiple rules). 4 DB tests (CRUD, cascade,
duplicate, GetRouteID). 3 gRPC tests (add+list, remove, validation).
2 end-to-end L7 tests (UA block, required header with allow/deny).

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

internal/l7/serve.go

@@ -26,6 +26,7 @@ type RouteConfig struct {
 	BackendTLS        bool
 	SendProxyProtocol bool
 	ConnectTimeout    time.Duration
+	Policies          []PolicyRule
 }
 
 // contextKey is an unexported type for context keys in this package.
@@ -74,10 +75,12 @@ func Serve(ctx context.Context, conn net.Conn, peeked []byte, route RouteConfig,
 		return fmt.Errorf("creating reverse proxy: %w", err)
 	}
 
-	// Wrap the handler to inject the real client IP into the request context.
+	// Build handler chain: context injection → L7 policies → reverse proxy.
+	var inner http.Handler = rp
+	inner = PolicyMiddleware(route.Policies, inner)
 	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		r = r.WithContext(context.WithValue(r.Context(), clientAddrKey, clientAddr))
-		rp.ServeHTTP(w, r)
+		inner.ServeHTTP(w, r)
 	})
 
 	// Serve HTTP on the TLS connection. Use HTTP/2 if negotiated,