fix(memory): add data integrity constraints to Fact model

- Change source_episode_ids from JSON to JSONB for PostgreSQL consistency
- Add unique constraint for global facts (project_id IS NULL)
- Add CHECK constraint ensuring reinforcement_count >= 1

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/models/memory/fact.py

@@ -18,9 +18,8 @@ from sqlalchemy import (
     Text,
     text,
 )
-from sqlalchemy.dialects.postgresql import UUID as PGUUID
+from sqlalchemy.dialects.postgresql import JSONB, UUID as PGUUID
 from sqlalchemy.orm import relationship
-from sqlalchemy.types import JSON
 
 from app.models.base import Base, TimestampMixin, UUIDMixin
 
@@ -61,8 +60,8 @@ class Fact(Base, UUIDMixin, TimestampMixin):
     # Confidence score (0.0 to 1.0)
     confidence = Column(Float, nullable=False, default=0.8, index=True)
 
-    # Source tracking: which episodes contributed to this fact (stored as JSON array of UUID strings)
-    source_episode_ids: Column[list] = Column(JSON, default=list, nullable=False)
+    # Source tracking: which episodes contributed to this fact (stored as JSONB array of UUID strings)
+    source_episode_ids: Column[list] = Column(JSONB, default=list, nullable=False)
 
     # Learning history
     first_learned = Column(DateTime(timezone=True), nullable=False)
@@ -86,6 +85,15 @@ class Fact(Base, UUIDMixin, TimestampMixin):
             unique=True,
             postgresql_where=text("project_id IS NOT NULL"),
         ),
+        # Unique constraint on triple for global facts (project_id IS NULL)
+        Index(
+            "ix_facts_unique_triple_global",
+            "subject",
+            "predicate",
+            "object",
+            unique=True,
+            postgresql_where=text("project_id IS NULL"),
+        ),
         # Query patterns
         Index("ix_facts_subject_predicate", "subject", "predicate"),
         Index("ix_facts_project_subject", "project_id", "subject"),
@@ -96,6 +104,10 @@ class Fact(Base, UUIDMixin, TimestampMixin):
             "confidence >= 0.0 AND confidence <= 1.0",
             name="ck_facts_confidence_range",
         ),
+        CheckConstraint(
+            "reinforcement_count >= 1",
+            name="ck_facts_reinforcement_positive",
+        ),
     )
 
     def __repr__(self) -> str: