Fix Polls Race Condition: Prevent Duplicate Votes

Problem

Users could cast multiple votes in single-select polls due to race condition issue:

Issue #1: Same Option Multiple Times by same user on same poll

User clicks same option rapidly → Multiple vote records created for identical choice
This issue can be seen in #1908

User A | Poll: "Lunch?" | Option: "Yes" 
Database: 3 duplicate rows ❌

Issue #2: Multiple Options in Single-Select Poll by same user

User rapidly clicks different options → Both votes registered in single-select poll

User A | Poll: "Approve?" (single-select)
Database: "Yes" ✓ and "No" ✓ (both recorded!) ❌

Root Cause: No database-level uniqueness enforcement. Application checks and inserts were separate operations, allowing concurrent requests to slip through.

Solution

Restructured from flat table to parent-child model with UNIQUE(poll_id, user_id) constraint.

Before (Flat Structure)

Raven Poll Vote: poll_id, user_id, option
→ Multiple rows per user possible ❌

After (Parent-Child Structure)

Raven Poll Vote: poll_id, user_id
  UNIQUE(poll_id, user_id) ✓
  └── Raven Poll Vote Selection (child): option

Database enforces: One vote per user per poll. Race conditions now fail at DB level.

Changes

Backend

1. New DocType: Raven Poll Vote Selection (child table for selected options)
2. Updated APIs: add_vote(), get_poll(), get_all_votes() - same structure - different operations internally
3. Updated DocType: Validations in before_insert(), counting from child table in update_poll_votes()
4. Unique Constraint: UNIQUE(poll_id, user_id) on Raven Poll Vote

Migration

1. Cleanup Patch: Removes existing duplicates (keeps oldest for same option, last modified for different options)
2. Migration Patch: Moves option data to child table Raven Poll Vote Selection
3. Constraint Patch: Adds unique constraint after cleanup
4. In "on_doctype_update" added check frappe.flags.in_install to only apply the unique constraint directly on fresh site install. Else it would be applied via patch.

Testing

Verified results on latest backup of prod.