feat: recreate Server Agent for premium VM management

Session-based agent with OpenAI Function Calling (9 tools).
Follows ddos-agent pattern: execute tools inside loop, feed results back to AI.
Includes D1 migration, session routing in openai-service, and doc updates.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

migrations/013_recreate_server_sessions.sql

@@ -0,0 +1,14 @@
+-- Recreate server management sessions table
+-- Previous migration 012 dropped this table when recommendation feature was removed
+-- Now recreated for premium VM management agent (no recommendation, management only)
+
+CREATE TABLE IF NOT EXISTS server_sessions (
+  user_id TEXT PRIMARY KEY,
+  status TEXT NOT NULL DEFAULT 'idle',
+  collected_info TEXT NOT NULL DEFAULT '{}',
+  messages TEXT NOT NULL DEFAULT '[]',
+  created_at INTEGER NOT NULL,
+  updated_at INTEGER NOT NULL,
+  expires_at INTEGER NOT NULL
+);
+CREATE INDEX IF NOT EXISTS idx_server_sessions_expires ON server_sessions(expires_at);