feat: add route-aware pricing estimates (#1695)

Salvaged from PR #1563 by @kshitijk4poor. Cherry-picked with authorship preserved.

- Route-aware pricing architecture replacing static MODEL_PRICING + heuristics
- Canonical usage normalization (Anthropic/OpenAI/Codex API shapes)
- Cache-aware billing (separate cache_read/cache_write rates)
- Cost status tracking (estimated/included/unknown/actual)
- OpenRouter live pricing via models API
- Schema migration v4→v5 with billing metadata columns
- Removed speculative forward-looking entries
- Removed cost display from CLI status bar
- Threaded OpenRouter metadata pre-warm

Co-authored-by: kshitij <82637225+kshitijk4poor@users.noreply.github.com>

tests/gateway/test_status_command.py

@@ -128,6 +128,13 @@ async def test_handle_message_persists_agent_token_counts(monkeypatch):
         session_entry.session_key,
         input_tokens=120,
         output_tokens=45,
+        cache_read_tokens=0,
+        cache_write_tokens=0,
         last_prompt_tokens=80,
         model="openai/test-model",
+        estimated_cost_usd=None,
+        cost_status=None,
+        cost_source=None,
+        provider=None,
+        base_url=None,
     )