feat(fusion_accounting_ai): bank_rec_prompt for AI re-rank step

Provider-agnostic system + user prompt builder for the confidence
scoring pipeline's Pass 3 (AI re-rank). Output contract is JSON with
"ranked" array; works with OpenAI, Claude, and local OpenAI-compatible
servers (LM Studio, Ollama).

Made-with: Cursor

fusion_accounting_ai/services/prompts/__init__.py

@@ -1,2 +1,3 @@
 from . import system_prompt
 from . import domain_prompts
+from . import bank_rec_prompt