fix: data integrity — deep-research result parsing + memory-extraction durability (#808)

Two independent data-integrity bugs:

- services/research/service.py: ResearchService.research() (the public deep-research
  API, re-exported from services/__init__) treated the handler return value as a
  dict (result.get("sources"/"summary"/...)), but call_research_service() returns a
  formatted markdown STRING -> AttributeError: str has no attribute get on EVERY
  successful call, making the API unusable for any non-error result. Now uses the
  string report as the summary and parses sources from the "### Sources" markdown
  section (section-bounded, URL-deduped), with a defensive dict branch for back-compat.

- services/memory/memory_extractor.py: extract_and_store guarded the vector-store
  find_similar/add calls only with the .healthy flag set ONCE at init. If the
  embedding/ChromaDB backend degraded LATER (OOM, evicted model, remote endpoint
  down), those calls raised, the exception escaped the dedup loop, skipped
  memory_manager.save(), and was swallowed by the outer try/except -> EVERY
  validated fact from the session was silently lost (the function docstring
  promises "never raised"). Now falls back to the existing text/fuzzy dedup so
  facts are still saved when the vector index is unavailable at runtime.

Tests: test_research_service.py, test_memory_extractor_vector_degraded.py.

services/memory/memory_extractor.py

@@ -303,9 +303,18 @@ async def extract_and_store(
             if not fact_text or len(fact_text) < 5:
                 continue
 
-            # Dedup: check vector similarity first (fast), then exact text match
+            # Dedup: check vector similarity first (fast), then exact text match.
+            # A runtime embedding/ChromaDB failure (backend OOM, model evicted,
+            # remote endpoint down) must not abort the whole batch — fall through
+            # to the text/fuzzy dedup below instead of losing every validated
+            # fact extracted this session. (`.healthy` is only set at init, so
+            # it does not catch failures that develop later.)
             if memory_vector and memory_vector.healthy:
-                existing_id = memory_vector.find_similar(fact_text, threshold=0.72)
+                try:
+                    existing_id = memory_vector.find_similar(fact_text, threshold=0.72)
+                except Exception as e:
+                    logger.warning(f"Memory dedup (vector) unavailable, using text fallback: {e}")
+                    existing_id = None
                 if existing_id:
                     logger.debug(f"Memory dedup (vector): '{fact_text[:50]}' matches {existing_id}")
                     continue
@@ -330,9 +339,14 @@ async def extract_and_store(
 
             existing.append(entry)
 
-            # Add to vector index
+            # Add to vector index. The JSON store (saved below) is the source of
+            # truth and the keyword path can still retrieve this entry, so a vector
+            # write failure must not drop the fact or abort the remaining batch.
             if memory_vector and memory_vector.healthy:
-                memory_vector.add(entry["id"], fact_text)
+                try:
+                    memory_vector.add(entry["id"], fact_text)
+                except Exception as e:
+                    logger.warning(f"Memory vector add failed for {entry['id']}: {e}")
 
             added += 1