feat: enrich master persona prompts (100-150 lines each) (#361)

* feat: add Buddhist master persona mode to AI chat

Users can now select a specific Buddhist master (智顗/慧能/玄奘/法藏/鸠摩罗什/印光/蕅益/虚云) in the chat UI. When selected:

- System prompt switches to that master's teaching style and doctrine
- RAG retrieval is scoped to the master's core scriptures (via text_id filtering)
- Citations follow the master's tradition-specific format

New files:
- backend/app/services/master_profiles.py: 8 master profiles with system prompts
- GET /api/chat/masters: list available masters for frontend selector

Modified:
- ChatRequest schema: added optional master_id field
- chat service: _prepare_chat/_build_llm_messages accept master_id
- similarity_search: scope_text_ids filtering for master-specific RAG
- Frontend: master selector dropdown above chat input

Powered by https://github.com/xr843/Master-skill v0.3 architecture.

* fix: CI failures — escape Chinese quotes in master prompt, nosec B608, fix hooks deps

* fix: escape all embedded ASCII quotes in master_profiles.py

* feat: enrich all 8 master prompts to 100-150 lines with teaching/voice/terms

Author: xr843