refactor: migrate from go-telegram-bot-api/v5 to go-telegram/bot

- Replaced tgbotapi with go-telegram/bot (zero-dependency, context-aware, Bot API 10.0)
- All handler methods now accept context.Context; threading ctx through all sends/edits
- Changed from function-based (NewMessage/NewEditMessageText/NewInlineKeyboardMarkup) to struct-param API (SendMessageParams/EditMessageTextParams/InlineKeyboardMarkup)
- Added colored buttons: DEL buttons in calendar use Style: 'danger' (red)
- Both polling and webhook modes preserved with new library patterns
- Context-based shutdown (signal.NotifyContext) replaces stop channel

go.sum

@@ -2,8 +2,8 @@ github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
 github.com/dustin/go-humanize v1.0.1 h1:GzkhY7T5VNhEkwH0PVJgjz+fX1rhBrR7pRT3mDkpeCY=
 github.com/dustin/go-humanize v1.0.1/go.mod h1:Mu1zIs6XwVuF/gI1OepvI0qD18qycQx+mFykh5fBlto=
-github.com/go-telegram-bot-api/telegram-bot-api/v5 v5.5.1 h1:wG8n/XJQ07TmjbITcGiUaOtXxdrINDz1b0J1w0SzqDc=
-github.com/go-telegram-bot-api/telegram-bot-api/v5 v5.5.1/go.mod h1:A2S0CWkNylc2phvKXWBBdD3K0iGnDBGbzRpISP2zBl8=
+github.com/go-telegram/bot v1.21.0 h1:Va/PbGc2vBDdv57GCUEEVV6ROlHWiC6SklJY9Hvhzps=
+github.com/go-telegram/bot v1.21.0/go.mod h1:i2TRs7fXWIeaceF3z7KzsMt/he0TwkVC680mvdTFYeM=
 github.com/google/pprof v0.0.0-20250317173921-a4b03ec1a45e h1:ijClszYn+mADRFY17kjQEVQ1XRhq2/JR1M3sGqeJoxs=
 github.com/google/pprof v0.0.0-20250317173921-a4b03ec1a45e/go.mod h1:boTsfXsheKC2y+lKOCMpSfarhxDeIzfZG1jqGcPl3cA=
 github.com/google/uuid v1.6.0 h1:NIvaJDMOsjHA8n1jAhLSgzrAzy1Hgr+hNrb57e+94F0=