feat: wire and persist coder_dlp_policy from provisioner to database - #25464

Adds a new coderd/dlppolicy package whose ForAgent helper loads the
agent's persisted DLP policy under dbauthz.AsSystemRestricted. Callers
have already authorized the request against the workspace; the policy
lookup is a follow-on system-internal read.

Four enforcement gates are wired in:

  * CLI peering. workspaceAgentClientCoordinate returns 403 when
    ssh_access is false. This is the single chokepoint for coder CLI
    peering, so the toggle is coarse and blocks ssh, port-forward, cp,
    and speedtest together.
  * Web terminal. workspaceapps.Server.workspaceAgentPTY returns 403
    before the WebSocket upgrade when web_terminal_access is false.
  * Dashboard Ports tab. workspaceapps.Server.proxyWorkspaceApp returns
    a "Blocked by workspace policy" HTML page when port_forwarding_access
    is false and the request resolves to a port view.
  * Dashboard app proxy. The same handler returns the same HTML page
    when the resolved app slug is not in allowed_applications.

User-facing strings say "workspace policy" and never include the
policy name; internal names and log fields keep "DLP". Browser-facing
denials render site.RenderStaticErrorPage; CLI and JSON callers get
codersdk.Response 403s.

The fix folded in here (originally adbdbfe) reads slugOrPort and
isPort from the signed appToken instead of app.PortInfo() /
app.AppSlugOrPort, because path-based app routing passes an empty
appurl.ApplicationURL into proxyWorkspaceApp and the parsed values are
not reliable on that path.