fix: use upsert to prevent IntegrityError during parallel search indexing

Replace delete-then-insert pattern with INSERT ... ON CONFLICT for
PostgreSQL search index operations. This fixes race conditions where
parallel entity indexing could cause UniqueViolationError on the
uix_search_index_permalink_project constraint.

Changes:
- Add index_item() override in PostgresSearchRepository with upsert
- Update bulk_index_items() to use ON CONFLICT (permalink, project_id)
- Add CREATE_POSTGRES_SEARCH_INDEX_PERMALINK DDL for test fixtures
- Add tests for upsert behavior on duplicate permalinks

Technical note: Use column-based ON CONFLICT syntax instead of
ON CONSTRAINT (which only works for table constraints, not indexes).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: phernandez <paul@basicmachines.co>

src/basic_memory/models/search.py

@@ -48,6 +48,15 @@ CREATE_POSTGRES_SEARCH_INDEX_METADATA = DDL("""
 CREATE INDEX IF NOT EXISTS idx_search_index_metadata_gin ON search_index USING gin(metadata jsonb_path_ops)
 """)
 
+# Partial unique index on (permalink, project_id) for non-null permalinks
+# This prevents duplicate permalinks per project and is used by upsert operations
+# in PostgresSearchRepository to handle race conditions during parallel indexing
+CREATE_POSTGRES_SEARCH_INDEX_PERMALINK = DDL("""
+CREATE UNIQUE INDEX IF NOT EXISTS uix_search_index_permalink_project
+ON search_index (permalink, project_id)
+WHERE permalink IS NOT NULL
+""")
+
 # Define FTS5 virtual table creation for SQLite only
 # This DDL is executed separately for SQLite databases
 CREATE_SEARCH_INDEX = DDL("""