Persist content strategy runtime state in DB-backed SharedStateBackend

[AJaySi]

Motivation

• Remove fragile in-memory state patterns used for AI polling/streaming and make runtime state robust across workers.
• Ensure tenant isolation by keying runtime records by authenticated user_id plus task/resource/cache identifiers.
• Add TTL/cleanup and clear not-found semantics so expired tasks/cache entries are handled consistently.

Description

• Introduces a DB-backed SharedStateBackend service (backend/services/shared_state_backend.py) implementing task lifecycle, latest-strategy refs, and short-lived streaming cache with TTL and cleanup.
• Adds SQLAlchemy models StrategyGenerationTaskState, LatestGeneratedStrategyState, and StreamingCacheState in backend/models/content_strategy_state_models.py (tenant-scoped keys + unique indexes).
• Replaces in-memory holders in endpoints: polling flow in ai_generation_endpoints.py now uses authenticated current_user, writes/reads task status via SharedStateBackend, persists latest strategy refs, and returns robust 404s for expired/missing tasks.
• Replaces streaming_cache in streaming_endpoints.py with the shared DB cache (SharedStateBackend) for strategic intelligence and keyword research flows, and registers the new models in DB init (backend/services/database.py) so tables are created.

Testing

• Static checks: python -m py_compile backend/services/shared_state_backend.py backend/api/content_planning/api/content_strategy/endpoints/ai_generation_endpoints.py backend/api/content_planning/api/content_strategy/endpoints/streaming_endpoints.py backend/models/content_strategy_state_models.py backend/tests/integration/test_shared_state_backend.py succeeded.
• Integration: ran cd backend && PYTHONPATH=/workspace/ALwrity/backend pytest -q tests/integration/test_shared_state_backend.py which exercises multi-worker semantics (separate SQLAlchemy sessions), TTL expiry and not-found behavior; all tests passed (4 passed).
• Notes: test run emits existing environment/dependency and SQLAlchemy deprecation warnings but the new integration tests passed.

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Codex Task