refactor(usage): extract indexing metering helper; address review round 2

Round-2 claude-review findings:

- 🟡 Base-class recursion invariant: documented on embed_with_usage /
  embed_batch_with_usage that a provider overriding embed()/embed_batch() to
  delegate to the *_with_usage variant MUST also override that variant, or the
  two recurse. (No recursion today; the shipped providers pair the overrides.)
- 🟡 Processor metering had no unit test: extracted the two-event recording
  into a module-level record_indexing_usage() helper and added
  tests/unit/test_processor_metering.py (value mapping, flag/zero-chunk no-ops,
  best-effort failure swallowed).
- 🟡 SonarQube hotspots (python:S5332) were 3 http:// URLs in the new test
  fixtures (mock hosts, never contacted) blocking the quality gate
  (new_security_hotspots_reviewed). Switched them to https:// so no hotspot is
  raised.
- 🟢 Zero-chunk guard: record_indexing_usage() no-ops when chunk_count == 0, so
  an empty document no longer writes zero-value billing rows.

Deferred (stated on the PR): Mistral x.index-or-0 sort key (pre-existing,
equivalent), CHANGELOG note for the Ollama /api/embed switch (CHANGELOG is
commitizen-generated from commit bodies, which document it), class-var
query_token_count (safe under the per-request instance pattern).

Deck #67.

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

tests/unit/providers/test_gateway_provider.py

@@ -331,8 +331,9 @@ def test_trailing_slash_base_url_normalized():
 async def test_gateway_embed_with_usage_forwards_after_bearer(monkeypatch):
     """embed_with_usage refreshes the bearer, then returns the (embedding,
     token_count) from the inherited OpenAI implementation."""
+    # https mock host (never contacted — the OpenAI client is patched below).
     provider = GatewayProvider(
-        base_url="http://gw:8083/v1", embedding_model="mistral/mistral-embed"
+        base_url="https://gw:8083/v1", embedding_model="mistral/mistral-embed"
     )
 
     order: list[str] = []
@@ -364,8 +365,9 @@ async def test_gateway_embed_with_usage_forwards_after_bearer(monkeypatch):
 
 async def test_gateway_embed_batch_with_usage_forwards_after_bearer(monkeypatch):
     """embed_batch_with_usage also refreshes the bearer before delegating."""
+    # https mock host (never contacted — the OpenAI client is patched below).
     provider = GatewayProvider(
-        base_url="http://gw:8083/v1", embedding_model="mistral/mistral-embed"
+        base_url="https://gw:8083/v1", embedding_model="mistral/mistral-embed"
     )
     ensured = {"n": 0}