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/services/memory/types.py

@@ -6,12 +6,17 @@ Core type definitions and interfaces for the Agent Memory System.
 
 from abc import ABC, abstractmethod
 from dataclasses import dataclass, field
-from datetime import datetime
+from datetime import UTC, datetime
 from enum import Enum
 from typing import Any
 from uuid import UUID
 
 
+def _utcnow() -> datetime:
+    """Get current UTC time as timezone-aware datetime."""
+    return datetime.now(UTC)
+
+
 class MemoryType(str, Enum):
     """Types of memory in the agent memory system."""
 
@@ -93,7 +98,7 @@ class MemoryItem:
 
     def get_age_seconds(self) -> float:
         """Get the age of this memory item in seconds."""
-        return (datetime.now() - self.created_at).total_seconds()
+        return (_utcnow() - self.created_at).total_seconds()
 
 
 @dataclass
@@ -106,14 +111,14 @@ class WorkingMemoryItem:
     key: str
     value: Any
     expires_at: datetime | None = None
-    created_at: datetime = field(default_factory=datetime.now)
-    updated_at: datetime = field(default_factory=datetime.now)
+    created_at: datetime = field(default_factory=_utcnow)
+    updated_at: datetime = field(default_factory=_utcnow)
 
     def is_expired(self) -> bool:
         """Check if this item has expired."""
         if self.expires_at is None:
             return False
-        return datetime.now() > self.expires_at
+        return _utcnow() > self.expires_at
 
 
 @dataclass
@@ -128,8 +133,8 @@ class TaskState:
     total_steps: int = 0
     progress_percent: float = 0.0
     context: dict[str, Any] = field(default_factory=dict)
-    started_at: datetime = field(default_factory=datetime.now)
-    updated_at: datetime = field(default_factory=datetime.now)
+    started_at: datetime = field(default_factory=_utcnow)
+    updated_at: datetime = field(default_factory=_utcnow)
 
 
 @dataclass