feat: add Battle.net username validator

[anonymort]

Summary

• Adds username availability checking for Battle.net via Overwatch player search API
• Uses generic_validate to check JSON response: empty array = available, non-empty = taken
• Includes BattleTag username pre-validation (3-12 chars, starts with letter, alphanumeric only)

Implementation Details

Endpoint: https://overwatch.blizzard.com/en-us/search/account-by-name/{username}

API behavior:

• 200 with [{...player data...}] → Username taken
• 200 with [] → Username available

File: user_scanner/user_scan/gaming/battlenet.py

Test Results

Username	Expected	Result
Faker	Taken	Taken
Blizzard	Taken	Taken
zzqnoexist	Available	Available
ab	Error (too short)	Error
123test	Error (starts with number)	Error

Caveats (documented in code)

• This checks Overwatch profiles specifically via the Blizzard player search
• A Battle.net account may exist without an Overwatch profile (and vice versa)
• BattleTag discriminators (#1234) are stripped if provided

Closes #97

[kaifcodec]

@anonymort Thanks for the PR. We’ll test it locally against edge cases, make any necessary adjustments, and then merge it.

[json-hunter07]

I don't know whether it is needed or not because Battle.net doesn't use unique usernames, they assign a tag by themselves at the end to make it unique.