Added notification batching with per-messenger rate limits

[nt0xa]

Bursts of events per payload are summarized into one message per window via batcher, and each messenger is rate limited according to its API limits.

[codecov]

Codecov Report

❌ Patch coverage is 54.28571% with 64 lines in your changes missing coverage. Please review.
✅ Project coverage is 44.06%. Comparing base (40326d9) to head (80469a8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/cmd/server/events.go	50.00%	14 Missing and 1 partial ⚠️
internal/modules/slack/notifier.go	0.00%	15 Missing ⚠️
internal/modules/lark/notifier.go	0.00%	13 Missing ⚠️
internal/modules/telegram/notifier.go	0.00%	13 Missing ⚠️
internal/cmd/server/run.go	0.00%	6 Missing ⚠️
internal/templates/templates.go	77.77%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #255      +/-   ##
==========================================
+ Coverage   43.65%   44.06%   +0.41%     
==========================================
  Files         216      216              
  Lines        8176     8305     +129     
==========================================
+ Hits         3569     3660      +91     
- Misses       4326     4360      +34     
- Partials      281      285       +4     

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