fix(indexing): unblock chunk indexing and gate reranker model load

Three defects surfaced during the first live ingest:

1. OpenSearch index mapping is dynamic:"strict", so the first chunk
   carrying metadata.section_heading was rejected with
   strict_dynamic_mapping_exception and zero chunks were indexed. The
   metadata and quality_flags sub-objects now opt into
   dynamic:true so chunker-specific keys insert without a fixed schema,
   while the top-level mapping stays strict.

2. hybrid_search.run_search called get_reranker() unconditionally, so
   every search - even search_mode=lexical with RERANKER_ENABLED=false
   - triggered a multi-GB FlagReranker download and hung the request.
   The loader is now lazy behind the reranker_enabled flag.

3. docker-compose x-common-env never forwarded RERANKER_ENABLED, so the
   container always defaulted to true regardless of .env. Added
   RERANKER_ENABLED: ${RERANKER_ENABLED:-true} to common-env.

After these fixes POST /search (lexical) returns HTTP 200 in ~50ms with
a ranked hit and citation; OpenSearch _count reports the indexed chunk.

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

app/indexing/opensearch_client.py

@@ -72,8 +72,12 @@ INDEX_SETTINGS: dict[str, Any] = {
             },
             "ocr_confidence": {"type": "float"},
             "language_hint": {"type": "keyword"},
-            "metadata": {"type": "object", "enabled": True},
-            "quality_flags": {"type": "object", "enabled": True},
+            # metadata + quality_flags carry chunker-specific keys
+            # (section_heading, table_index, low_ocr_confidence, ...) that we do
+            # not want to enumerate up-front. Top-level mapping is dynamic:
+            # "strict", but these two sub-objects opt-in to dynamic insertion.
+            "metadata": {"type": "object", "enabled": True, "dynamic": True},
+            "quality_flags": {"type": "object", "enabled": True, "dynamic": True},
             "created_at": {"type": "date"},
         },
     },