gateway: capture real provider-reported cost (openrouter usage accounting)

Cost displays were estimates from a pricing table; on OpenRouter the
status bar never reflected what was actually charged. Now cost is
provider-REPORTED only, end to end:

- OpenRouter requests carry usage:{include:true} (profile + legacy
  transport paths); the response usage.cost field (credits, 1:1 USD)
  is captured per call into agent.session_actual_cost_usd and
  persisted to the sessions DB actual_cost_usd column (NULL-safe:
  unreported calls never touch the stored value).
- Nous keeps its x-nous-credits-* header capture; the header delta
  now surfaces as the session's real cost via real_session_cost_usd.
- Providers that report nothing accumulate NOTHING: cost fields stay
  absent/None (the TUI hides its cost segment), never a fabricated
  $0.00 and never an estimate. _get_usage, gateway /usage and the
  CLI usage page all switched off estimate_usage_cost for display.
- Per-model session accumulator (session_model_usage) records real
  per-call counts and provider-reported cost per model.

agent/agent_init.py

@@ -1620,6 +1620,12 @@ def init_agent(
     agent.session_cache_write_tokens = 0
     agent.session_reasoning_tokens = 0
     agent.session_estimated_cost_usd = 0.0
+    # Provider-REPORTED cost only (e.g. OpenRouter usage.cost). None means
+    # "nothing reported" — distinct from a real $0.00.
+    agent.session_actual_cost_usd = None
+    # Per-model session usage rows for /usage: {model: {calls, input, output,
+    # cache_read, cache_write, cost_usd|None}}.
+    agent.session_model_usage = {}
     agent.session_cost_status = "unknown"
     agent.session_cost_source = "none"