fix(dao): ensure find and create functions thread safe

[qwqcode]

Due to the fact that find and create operations are not atomic, potential concurrency issues may arise, leading to multiple executions of the create operation (while the expectation is to execute create only once). This results in the possibility of multiple pages or similar entries in the data table. This PR addresses concurrency issues by utilizing mutex locks at the underlying level via singleflight.

We provide a tool for merging (due to this issue) duplicate pages. To run this tool when Artalk starts, add the environment variable ATK_DB_MIGRATOR_FUNC_MERGE_PAGES=1. Before executing this tool, be sure to backup the original data.

[codecov]

Codecov Report

Attention: Patch coverage is 25.96154% with 77 lines in your changes are missing coverage. Please review.

Project coverage is 27.62%. Comparing base (fbd1cd9) to head (c8d2df3).

❗ Current head c8d2df3 differs from pull request most recent head f18070e. Consider uploading reports for the commit f18070e to get more accurate results

Files	Patch %	Lines
internal/dao/migrate.go	4.44%	42 Missing and 1 partial ⚠️
internal/utils/cli.go	0.00%	18 Missing ⚠️
internal/dao/query_find_create.go	60.97%	13 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #845      +/-   ##
==========================================
- Coverage   27.78%   27.62%   -0.17%     
==========================================
  Files         181      182       +1     
  Lines        7183     7266      +83     
==========================================
+ Hits         1996     2007      +11     
- Misses       5069     5137      +68     
- Partials      118      122       +4     

Flag	Coverage Δ
go	27.62% <25.96%> (-0.17%)	⬇️

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