feat(usage): rename metrics → tokens_embedded/pages_embedded + export token cost to Prometheus

Billing product model finalized (Deck #281): bill pages externally, record
tokens internally. Rename the data-plane metric literals to match the now-
canonical contract (Deck #284) — the control plane's METRIC_EVENT_NAMES is
already renamed, so the old names would be unmapped and never sync to Stripe.

Rename (values unchanged):
- embeddings_queries → tokens_embedded (value = real token count, already
  emitted by this PR; the unit upstream providers bill on).
- pages_chunks → pages_embedded (value kept as len(chunk_texts) interim;
  TODO(#282): real normalized "pages indexed" count — real pages for paginated
  types, chars/tokens-per-page constant otherwise — is deferred to the
  instrumentation card, this only lands the name/contract).
- All literals, log strings, docstrings, comments, the migration comment, and
  tests renamed; grep confirms zero old strings remain.

Observability (new): export embedding token cost to Prometheus as
astrolabe_embedding_tokens_total{provider,operation} (operation = index|query)
so the billed cost unit is visible in Grafana, not just the per-tenant billing
DB. Dedicated counter (doesn't inflate the existing chunk/request metrics) and
always-on (independent of USAGE_METERING_ENABLED, so OSS/self-host gets it).
Wired on both the indexing batch embed and the search query embed (query inside
the per-request cache-miss branch, so reused embeddings aren't double-counted).

Note: the rename orphans any pre-existing embeddings_queries/pages_chunks rows
in tenant app DBs (CP no longer maps them) — acceptable; pipeline is inert with
throwaway dev/sandbox data.

Deck #284 (folded into PR #875).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/unit/server/test_semantic_metering.py

@@ -1,6 +1,6 @@
 """Unit tests for the search-path usage-metering helper (Deck #67).
 
-``record_search_usage`` records the billable ``embeddings_queries`` event for a
+``record_search_usage`` records the billable ``tokens_embedded`` event for a
 semantic search. These pin the value mapping (query token count), the flag-off
 no-op, the doc_types metadata bounding, and the best-effort failure path —
 covering the server-tool metering wiring without standing up the full
@@ -38,7 +38,7 @@ async def test_records_query_token_count(store_spy):
 
     store_spy.record_usage_event.assert_awaited_once()
     kwargs = store_spy.record_usage_event.await_args.kwargs
-    assert kwargs["metric"] == "embeddings_queries"
+    assert kwargs["metric"] == "tokens_embedded"
     assert kwargs["value"] == 42
     assert kwargs["enabled"] is True
     assert kwargs["metadata"]["user_id"] == "alice"