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>

nextcloud_mcp_server/providers/base.py

@@ -73,7 +73,7 @@ class Provider(ABC):
         Returns ``(embedding, token_count)``. The default delegates to
         :meth:`embed` and estimates the tokens; providers that surface real
         usage from their embedding response override this. Used by the
-        usage-metering hooks (Deck #67) to bill ``embeddings_queries`` by
+        usage-metering hooks (Deck #67) to bill ``tokens_embedded`` by
         tokens rather than by operation count.
 
         IMPORTANT (recursion invariant): this default calls ``self.embed``. A

nextcloud_mcp_server/providers/mistral.py

@@ -143,7 +143,7 @@ class MistralProvider(Provider):
         Returns ``(embeddings, total_tokens)`` where ``total_tokens`` is the
         sum of ``response.usage.total_tokens`` across the ``BATCH_SIZE`` sub-
         requests (the unit Mistral bills on). Used by the usage-metering hooks
-        to record ``embeddings_queries`` by tokens (Deck #67).
+        to record ``tokens_embedded`` by tokens (Deck #67).
         """
         if not self.supports_embeddings:
             raise NotImplementedError(_NO_EMBEDDING_MODEL_MSG)