feat(02-01): backend prompts router with 4 CRUD endpoints

- GET /api/prompts — list all prompts with modified/default flag
- GET /api/prompts/{name} — read prompt content + required variables
- PUT /api/prompts/{name} — save modified prompt with validation
- POST /api/prompts/{name}/reset — restore prompt to default
- Lazy PromptService init to handle lifespan directory creation
- Router registered in main.py before SPA catch-all

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/main.py

@@ -14,7 +14,7 @@ from fastapi.responses import FileResponse
 from fastapi.staticfiles import StaticFiles
 
 from backend.config import CAMPAIGNS_PATH, CONFIG_PATH, OUTPUTS_PATH, PROMPTS_PATH
-from backend.routers import calendar, export, generate, settings
+from backend.routers import calendar, export, generate, prompts, settings
 
 
 # ---------------------------------------------------------------------------
@@ -91,6 +91,7 @@ app.include_router(calendar.router)
 app.include_router(generate.router)
 app.include_router(export.router)
 app.include_router(settings.router)
+app.include_router(prompts.router)
 
 
 # ---------------------------------------------------------------------------