feat(usage): meter embedding tokens as embeddings_queries on both paths

embeddings_queries now records the embedding request's token count (the unit
upstream providers bill on) instead of an operation count, and fires on the
indexing path too. Previously only semantic search recorded it (value=1), so a
re-indexing run produced no embeddings_queries events at all — only pages_chunks.

- Provider layer: additive embed_with_usage / embed_batch_with_usage surface the
  per-request token count (Mistral/OpenAI usage.total_tokens, Bedrock Titan
  inputTextTokenCount, Ollama prompt_eval_count); a char-based estimate is the
  fallback (Simple, and any provider/response without a token field). Gateway and
  EmbeddingService forward through. The count travels as a return value / a
  per-request SearchAlgorithm attribute — never on the singleton — so concurrent
  indexing + search can't mis-attribute bills.
- Indexing (vector/processor.py): records embeddings_queries (value=batch tokens)
  alongside the existing pages_chunks event.
- Search (server/semantic.py): value is now the query embedding's token count,
  relayed from BM25HybridSearchAlgorithm via query_token_count.

The astrolabe_embeddings_queries Stripe meter (sum aggregation) now sums tokens
with no CP/Terraform change. The meter "queries"->tokens naming/unit
clarification (homelab-terraform #254) + CP rollup/portal copy is a follow-up.

Deck #67.

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

tests/unit/providers/test_openai.py

@@ -280,6 +280,46 @@ async def test_openai_empty_batch():
     assert embeddings == []
 
 
+def _embed_item(embedding, index):
+    item = MagicMock()
+    item.embedding = embedding
+    item.index = index
+    return item
+
+
+@pytest.mark.unit
+async def test_openai_embed_batch_with_usage_reports_tokens(mock_openai_client):
+    """embed_batch_with_usage returns the response's total_tokens."""
+    response = MagicMock()
+    response.data = [_embed_item([0.1, 0.2], 0), _embed_item([0.3, 0.4], 1)]
+    response.usage = MagicMock(total_tokens=9)
+    mock_openai_client.embeddings.create = AsyncMock(return_value=response)
+
+    provider = OpenAIProvider(
+        api_key="test-key", embedding_model="text-embedding-3-small"
+    )
+    embeddings, tokens = await provider.embed_batch_with_usage(["a", "b"])
+
+    assert embeddings == [[0.1, 0.2], [0.3, 0.4]]
+    assert tokens == 9
+
+
+@pytest.mark.unit
+async def test_openai_with_usage_estimates_when_usage_absent(mock_openai_client):
+    """Missing usage falls back to the char-based estimate."""
+    response = MagicMock()
+    response.data = [_embed_item([0.1], 0)]
+    response.usage = None
+    mock_openai_client.embeddings.create = AsyncMock(return_value=response)
+
+    provider = OpenAIProvider(
+        api_key="test-key", embedding_model="text-embedding-3-small"
+    )
+    _, tokens = await provider.embed_with_usage("abcdefgh")  # 8 chars → 2 tokens
+
+    assert tokens == 2
+
+
 @pytest.mark.unit
 async def test_openai_close(mock_openai_client):
     """Test OpenAI client close."""