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/__init__.py

@@ -8,6 +8,19 @@ from app.core.database import Base
 
 from .base import TimestampMixin, UUIDMixin
 
+# Memory system models
+from .memory import (
+    ConsolidationStatus,
+    ConsolidationType,
+    Episode,
+    EpisodeOutcome,
+    Fact,
+    MemoryConsolidationLog,
+    Procedure,
+    ScopeType,
+    WorkingMemory,
+)
+
 # OAuth models (client mode - authenticate via Google/GitHub)
 from .oauth_account import OAuthAccount
 
@@ -37,7 +50,14 @@ __all__ = [
     "AgentInstance",
     "AgentType",
     "Base",
+    # Memory models
+    "ConsolidationStatus",
+    "ConsolidationType",
+    "Episode",
+    "EpisodeOutcome",
+    "Fact",
     "Issue",
+    "MemoryConsolidationLog",
     "OAuthAccount",
     "OAuthAuthorizationCode",
     "OAuthClient",
@@ -46,11 +66,14 @@ __all__ = [
     "OAuthState",
     "Organization",
     "OrganizationRole",
+    "Procedure",
     "Project",
+    "ScopeType",
     "Sprint",
     "TimestampMixin",
     "UUIDMixin",
     "User",
     "UserOrganization",
     "UserSession",
+    "WorkingMemory",
 ]