feat: LLM Validation section — 9 API endpoints + 3 Svelte pages + Playwright optimization

Backend:
- /api/validation/ CRUD for validation tasks (policies)
- Run history with 6 filters + pagination
- Alembic migrations (provider_id, policy_id)
- 28 pytest tests

Frontend:
- /validation — task list page with status filters + CRUD
- /validation/[id] — task config (Config + History tabs)
- /validation/history — run history with metrics
- Sidebar nav entry under Operations
- i18n en/ru

Playwright (ScreenshotService):
- Removed 25s hardcoded sleeps → conditional waits
- CDP fallback to full_page screenshot
- Total timeout enforcement (120s)
- Debug screenshots → temp dir with cleanup

QA fixes:
- Debug screenshot accumulation in production (tempdir + rmtree)
- Frontend API payload field name mismatch
- History issues field reference fix
- delete_runs scoped by policy_id

Belief protocol:
- @RATIONALE/@REJECTED on 27 C4+ contracts

Closes: VALIDATION-REWORK-2026

backend/src/app.py

@@ -44,6 +44,7 @@ from .api.routes import (
     storage,
     tasks,
     translate,
+    validation,
 )
 from .core.auth.security import get_password_hash
 from .core.cot_logger import seed_trace_id
@@ -217,6 +218,7 @@ app.add_middleware(TraceContextMiddleware)
 # @RELATION DEPENDS_ON -> [ReportsRouter]
 # @RELATION DEPENDS_ON -> [LlmRoutes]
 # @RELATION DEPENDS_ON -> [CleanReleaseV2Api]
+# @RELATION DEPENDS_ON -> [ValidationRoutes]
 # Include API routes
 app.include_router(auth.router)
 app.include_router(admin.router)
@@ -239,6 +241,7 @@ app.include_router(profile.router)
 app.include_router(dataset_review.router)
 app.include_router(health.router)
 app.include_router(translate.router)
+app.include_router(validation.router, prefix="/api/validation", tags=["Validation"])
 # #endregion API_Routes
 # #region api.include_routers [C:1] [TYPE Action] [SEMANTICS routes, registration, api]
 # @BRIEF Registers all API routers with the FastAPI application.