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/db/seed.go

@@ -31,7 +31,7 @@ func (s *Store) Seed(listeners []config.Listener, fw config.Firewall) error {
 			if mode == "" {
 				mode = "l4"
 			}
-			_, err := tx.Exec(
+			routeResult, err := tx.Exec(
 				`INSERT INTO routes (listener_id, hostname, backend, mode, tls_cert, tls_key, backend_tls, send_proxy_protocol)
 				 VALUES (?, ?, ?, ?, ?, ?, ?, ?)`,
 				listenerID, strings.ToLower(r.Hostname), r.Backend,
@@ -40,6 +40,18 @@ func (s *Store) Seed(listeners []config.Listener, fw config.Firewall) error {
 			if err != nil {
 				return fmt.Errorf("seeding route %q on listener %q: %w", r.Hostname, l.Addr, err)
 			}
+
+			if len(r.L7Policies) > 0 {
+				routeID, _ := routeResult.LastInsertId()
+				for _, p := range r.L7Policies {
+					if _, err := tx.Exec(
+						"INSERT INTO l7_policies (route_id, type, value) VALUES (?, ?, ?)",
+						routeID, p.Type, p.Value,
+					); err != nil {
+						return fmt.Errorf("seeding l7 policy on route %q: %w", r.Hostname, err)
+					}
+				}
+			}
 		}
 	}