feat(backend): V281.0 - Vision-First OCR, Visuals robustness (Pow fix), and Rigid Graph Identification

V280.0: Robust Math Validation Fixes & Model Lockdown (RCE Protection, Multiline Regex, Soft Fail)

QA Cycle 2.1: Data Slicing, Hotfix refinement, Firestore fix, and UI Safety updates

Critical QA Fixes: Bug 1-4 (Data Isolation, Hotfix Sanity, History Save, Visuals Stability)

V5.10.4: History Tier Fix (admin_unlimited) & Vision Extractor Graph Descriptions

V5.10.3: History Debugging & Graph Anti-Hallucination Prompt Fix

V5.10.2: Fix kwargs collision in smart_solve - pop explicit keys before spread

Implement SymPy Algebraic Verification & Self-Correction - Resolved critical regression and auth

Fix Digital Binder history saving - Parameter propagation & logging

Implement Digital Binder (History) - Conditional saving for premium users

feat: Implement Check Solution logic with score and structured mistakes

feat: AI Assessment Analytics Infrastructure

feat: multi-image support for V2 endpoint

V309.5: Rollback to HOTFIX + OCR Anti-Truncation + Abs Fix

V307.0: Polygraph Restore + Math Syntax Pre-processor + Graph Polish Sync

Hotfix: SymPy Parse Error bypass

V5.8.5: Enhanced 5-unit math (3D Vectors, Solid Geom, Optimization) + Placeholder Fixes + HF Space compatibility

Fix: Clean production deployment with sse-starlette