Add content strategy state models from PR #433

New models for managing content strategy runtime state:
- StrategyGenerationTaskState: Task lifecycle/status for polling-based AI generation
- LatestGeneratedStrategyState: References to latest generated strategy per user/resource
- StreamingCacheState: Short-lived streaming cache entries with TTL semantics

These models provide persistent state management for content strategy operations.

backend/models/content_strategy_state_models.py

@@ -0,0 +1,57 @@
+"""Shared persistent state models for content strategy runtime state."""
+
+from datetime import datetime
+
+from sqlalchemy import Column, Integer, String, DateTime, JSON, Index
+
+from models.enhanced_strategy_models import Base
+
+
+class StrategyGenerationTaskState(Base):
+    """Stores task lifecycle/status for polling-based AI generation."""
+
+    __tablename__ = "strategy_generation_task_state"
+
+    id = Column(Integer, primary_key=True)
+    user_id = Column(String(255), nullable=False, index=True)
+    task_id = Column(String(255), nullable=False, unique=True, index=True)
+    status_payload = Column(JSON, nullable=False)
+    expires_at = Column(DateTime, nullable=False, index=True)
+    created_at = Column(DateTime, default=datetime.utcnow, nullable=False)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
+
+
+class LatestGeneratedStrategyState(Base):
+    """Stores references to latest generated strategy payload per user/resource."""
+
+    __tablename__ = "latest_generated_strategy_state"
+
+    id = Column(Integer, primary_key=True)
+    user_id = Column(String(255), nullable=False, index=True)
+    resource_id = Column(String(255), nullable=False, default="comprehensive", index=True)
+    strategy_payload = Column(JSON, nullable=False)
+    expires_at = Column(DateTime, nullable=False, index=True)
+    created_at = Column(DateTime, default=datetime.utcnow, nullable=False)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
+
+    __table_args__ = (
+        Index("ix_latest_generated_strategy_user_resource", "user_id", "resource_id", unique=True),
+    )
+
+
+class StreamingCacheState(Base):
+    """Stores short-lived streaming cache entries with TTL semantics."""
+
+    __tablename__ = "streaming_cache_state"
+
+    id = Column(Integer, primary_key=True)
+    user_id = Column(String(255), nullable=False, index=True)
+    cache_key = Column(String(255), nullable=False, index=True)
+    cache_payload = Column(JSON, nullable=False)
+    expires_at = Column(DateTime, nullable=False, index=True)
+    created_at = Column(DateTime, default=datetime.utcnow, nullable=False)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
+
+    __table_args__ = (
+        Index("ix_streaming_cache_user_key", "user_id", "cache_key", unique=True),
+    )