Switch from telegram_bot_username to telegram_notifications in /config endpoint

[paskal]

Bot username is returned as an answer to subscribe request, so knowing it in advance is unnecessary. Needed for #830.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +4.25 :tada:

Comparison is base (d3fdd7b) 58.63% compared to head (f3927fc) 62.88%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1648 +/- ## ========================================== + Coverage 58.63% 62.88% +4.25% ========================================== Files 128 136 +8 Lines 2877 3387 +510 Branches 737 804 +67 ========================================== + Hits 1687 2130 +443 Misses 1065 1065 - Partials 125 192 +67 ``` | [Impacted Files](https://app.codecov.io/gh/umputun/remark42/pull/1648?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Umputun) | Coverage Δ | | |---|---|---| | [frontend/apps/remark42/app/common/static-store.ts](https://app.codecov.io/gh/umputun/remark42/pull/1648?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Umputun#diff-ZnJvbnRlbmQvYXBwcy9yZW1hcms0Mi9hcHAvY29tbW9uL3N0YXRpYy1zdG9yZS50cw==) | `100.00% <ø> (ø)` | | | [frontend/apps/remark42/app/common/types.ts](https://app.codecov.io/gh/umputun/remark42/pull/1648?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Umputun#diff-ZnJvbnRlbmQvYXBwcy9yZW1hcms0Mi9hcHAvY29tbW9uL3R5cGVzLnRz) | `100.00% <ø> (ø)` | | | [frontend/packages/api/clients/public.ts](https://app.codecov.io/gh/umputun/remark42/pull/1648?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Umputun#diff-ZnJvbnRlbmQvcGFja2FnZXMvYXBpL2NsaWVudHMvcHVibGljLnRz) | `92.81% <100.00%> (ø)` | | ... and [7 files with indirect coverage changes](https://app.codecov.io/gh/umputun/remark42/pull/1648/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Umputun)

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[github-actions[bot]]

size-limit report 📦

Path	Size
public/embed.mjs	2.04 KB (0%)
public/remark.mjs	73.34 KB (-0.02% 🔽)
public/remark.css	8.12 KB (0%)
public/last-comments.mjs	35.92 KB (-0.03% 🔽)
public/last-comments.css	3.72 KB (+0.03% 🔺)
public/deleteme.mjs	12.12 KB (-0.07% 🔽)
public/counter.mjs	749 B (0%)