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_session.py

@@ -703,5 +703,15 @@ class TestLastPromptTokens:
         store.update_session("k1", model="openai/gpt-5.4")
 
         store._db.update_token_counts.assert_called_once_with(
-            "s1", 0, 0, model="openai/gpt-5.4"
+            "s1",
+            input_tokens=0,
+            output_tokens=0,
+            cache_read_tokens=0,
+            cache_write_tokens=0,
+            estimated_cost_usd=None,
+            cost_status=None,
+            cost_source=None,
+            billing_provider=None,
+            billing_base_url=None,
+            model="openai/gpt-5.4",
         )