feat(aia): Integrate PromptService for 10 AI agents

Features:
- Migrate 10 agent prompts from hardcoded to database
- Add grayscale preview support (DRAFT/ACTIVE distribution)
- Implement 3-tier fallback (DB -> Cache -> Hardcoded)
- Add version management and rollback capability

Files changed:
- backend/scripts/migrate-aia-prompts.ts (new migration script)
- backend/src/common/prompt/prompt.fallbacks.ts (add AIA fallbacks)
- backend/src/modules/aia/services/agentService.ts (integrate PromptService)
- backend/src/modules/aia/services/conversationService.ts (pass userId)
- backend/src/modules/aia/types/index.ts (fix AgentStage type)

Documentation:
- docs/03-业务模块/AIA-AI智能问答/06-开发记录/2026-01-18-Prompt管理系统集成.md
- docs/02-通用能力层/00-通用能力层清单.md (add FileCard, Prompt management)
- docs/00-系统总体设计/00-系统当前状态与开发指南.md (update to v3.6)

Prompt codes:
- AIA_SCIENTIFIC_QUESTION, AIA_PICO_ANALYSIS, AIA_TOPIC_EVALUATION
- AIA_OUTCOME_DESIGN, AIA_CRF_DESIGN, AIA_SAMPLE_SIZE
- AIA_PROTOCOL_WRITING, AIA_METHODOLOGY_REVIEW
- AIA_PAPER_POLISH, AIA_PAPER_TRANSLATE

Tested: Migration script executed, all 10 prompts inserted successfully

backend/verify_functions.ts

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