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/working_memory.py

@@ -0,0 +1,58 @@
+# app/models/memory/working_memory.py
+"""
+Working Memory database model.
+
+Stores ephemeral key-value data for active sessions with TTL support.
+Used as database backup when Redis is unavailable.
+"""
+
+from sqlalchemy import Column, DateTime, Enum, Index, String
+from sqlalchemy.dialects.postgresql import JSONB
+
+from app.models.base import Base, TimestampMixin, UUIDMixin
+
+from .enums import ScopeType
+
+
+class WorkingMemory(Base, UUIDMixin, TimestampMixin):
+    """
+    Working memory storage table.
+
+    Provides database-backed working memory as fallback when
+    Redis is unavailable. Supports TTL-based expiration.
+    """
+
+    __tablename__ = "working_memory"
+
+    # Scoping
+    scope_type: Column[ScopeType] = Column(
+        Enum(ScopeType),
+        nullable=False,
+        index=True,
+    )
+    scope_id = Column(String(255), nullable=False, index=True)
+
+    # Key-value storage
+    key = Column(String(255), nullable=False)
+    value = Column(JSONB, nullable=False)
+
+    # TTL support
+    expires_at = Column(DateTime(timezone=True), nullable=True, index=True)
+
+    __table_args__ = (
+        # Primary lookup: scope + key
+        Index(
+            "ix_working_memory_scope_key",
+            "scope_type",
+            "scope_id",
+            "key",
+            unique=True,
+        ),
+        # For cleanup of expired entries
+        Index("ix_working_memory_expires", "expires_at"),
+        # For listing all keys in a scope
+        Index("ix_working_memory_scope_list", "scope_type", "scope_id"),
+    )
+
+    def __repr__(self) -> str:
+        return f"<WorkingMemory {self.scope_type.value}:{self.scope_id}:{self.key}>"