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/migrations.go

@@ -45,6 +45,19 @@ ALTER TABLE routes ADD COLUMN send_proxy_protocol INTEGER NOT NULL DEFAULT 0;`,
 		Name:    "add_listener_max_connections",
 		SQL:     `ALTER TABLE listeners ADD COLUMN max_connections INTEGER NOT NULL DEFAULT 0;`,
 	},
+	{
+		Version: 4,
+		Name:    "create_l7_policies_table",
+		SQL: `
+CREATE TABLE IF NOT EXISTS l7_policies (
+	id       INTEGER PRIMARY KEY,
+	route_id INTEGER NOT NULL REFERENCES routes(id) ON DELETE CASCADE,
+	type     TEXT NOT NULL CHECK(type IN ('block_user_agent', 'require_header')),
+	value    TEXT NOT NULL,
+	UNIQUE(route_id, type, value)
+);
+CREATE INDEX IF NOT EXISTS idx_l7_policies_route ON l7_policies(route_id);`,
+	},
 }
 
 // Migrate runs all unapplied migrations sequentially.