feat(security): harden gateway boundaries and usage policies

Consolidate subscription usage policy enforcement, privacy-safe persistence, and gateway security hardening into one reviewable change.

Includes bounded HTTP and execution envelopes, header and protocol guards, DNS and relay validation, authentication and secret projection hardening, secure backup/install paths, and regression coverage.

crates/aether-data/runtime/schema/drivers/mysql/baseline/004_proxy_nodes.sql

@@ -14,7 +14,7 @@ CREATE TABLE IF NOT EXISTS proxy_nodes (
     is_manual TINYINT(1) NOT NULL DEFAULT 0,
     proxy_url VARCHAR(500),
     proxy_username VARCHAR(255),
-    proxy_password VARCHAR(500),
+    proxy_password TEXT,
     created_at BIGINT NOT NULL,
     updated_at BIGINT NOT NULL,
     remote_config TEXT,

crates/aether-data/runtime/schema/drivers/postgres/baseline/001_types_and_tables.sql

@@ -706,7 +706,7 @@ CREATE TABLE IF NOT EXISTS public.proxy_nodes (
     is_manual boolean DEFAULT false NOT NULL,
     proxy_url character varying(500),
     proxy_username character varying(255),
-    proxy_password character varying(500),
+    proxy_password text,
     created_at timestamp with time zone DEFAULT CURRENT_TIMESTAMP NOT NULL,
     updated_at timestamp with time zone DEFAULT CURRENT_TIMESTAMP NOT NULL,
     remote_config json,