feat(memory): add database schema and storage layer (Issue #88)

Add SQLAlchemy models for the Agent Memory System:
- WorkingMemory: Key-value storage with TTL for active sessions
- Episode: Experiential memories from task executions
- Fact: Semantic knowledge triples with confidence scores
- Procedure: Learned skills and procedures with success tracking
- MemoryConsolidationLog: Tracks consolidation jobs between memory tiers

Create enums for memory system:
- ScopeType: global, project, agent_type, agent_instance, session
- EpisodeOutcome: success, failure, partial
- ConsolidationType: working_to_episodic, episodic_to_semantic, etc.
- ConsolidationStatus: pending, running, completed, failed

Add Alembic migration (0005) for all memory tables with:
- Foreign key relationships to projects, agent_instances, agent_types
- Comprehensive indexes for query patterns
- Unique constraints for key lookups and triple uniqueness
- Vector embedding column placeholders (Text fallback until pgvector enabled)

Fix timezone-naive datetime.now() in types.py TaskState (review feedback)

Includes 30 unit tests for models and enums.

Closes #88

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

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

backend/app/models/memory/__init__.py

@@ -0,0 +1,32 @@
+# app/models/memory/__init__.py
+"""
+Memory System Database Models.
+
+Provides SQLAlchemy models for the Agent Memory System:
+- WorkingMemory: Key-value storage with TTL
+- Episode: Experiential memories
+- Fact: Semantic knowledge triples
+- Procedure: Learned skills
+- MemoryConsolidationLog: Consolidation job tracking
+"""
+
+from .consolidation import MemoryConsolidationLog
+from .enums import ConsolidationStatus, ConsolidationType, EpisodeOutcome, ScopeType
+from .episode import Episode
+from .fact import Fact
+from .procedure import Procedure
+from .working_memory import WorkingMemory
+
+__all__ = [
+    # Enums
+    "ConsolidationStatus",
+    "ConsolidationType",
+    # Models
+    "Episode",
+    "EpisodeOutcome",
+    "Fact",
+    "MemoryConsolidationLog",
+    "Procedure",
+    "ScopeType",
+    "WorkingMemory",
+]