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/policy.go

@@ -0,0 +1,38 @@
+package l7
+
+import (
+	"net/http"
+	"strings"
+)
+
+// PolicyRule defines an L7 blocking policy.
+type PolicyRule struct {
+	Type  string // "block_user_agent" or "require_header"
+	Value string
+}
+
+// PolicyMiddleware returns an http.Handler that evaluates L7 policies
+// before delegating to next. Returns HTTP 403 if any policy blocks.
+// If policies is empty, returns next unchanged.
+func PolicyMiddleware(policies []PolicyRule, next http.Handler) http.Handler {
+	if len(policies) == 0 {
+		return next
+	}
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		for _, p := range policies {
+			switch p.Type {
+			case "block_user_agent":
+				if strings.Contains(r.UserAgent(), p.Value) {
+					w.WriteHeader(http.StatusForbidden)
+					return
+				}
+			case "require_header":
+				if r.Header.Get(p.Value) == "" {
+					w.WriteHeader(http.StatusForbidden)
+					return
+				}
+			}
+		}
+		next.ServeHTTP(w, r)
+	})
+}