feat: Add project_id to Relation and Observation for efficient project-scoped queries

Denormalizes project_id onto Relation and Observation tables to enable
efficient project-scoped queries without joins. Migration backfills
from associated entity and adds pg_trgm extension with GIN indexes
for fuzzy link resolution on PostgreSQL.

🤖 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/repository/relation_repository.py

@@ -117,6 +117,7 @@ class RelationRepository(Repository[Relation]):
         # Convert Relation objects to dicts for insert
         values = [
             {
+                "project_id": r.project_id if r.project_id else self.project_id,
                 "from_id": r.from_id,
                 "to_id": r.to_id,
                 "to_name": r.to_name,