feat(channels): add Telegram and make all alert channels configurable from Settings

[gnacho]

Adds a Telegram alert channel (Bot API sendMessage) and makes every alert channel configurable from Settings.

What changed:

• New Telegram channel: bot token + chat id, same alert pipeline as the other channels (i18n ES/EN templates, severity prefix).
• ntfy/Gotify sends hardened: network errors no longer leak the topic or the bot token, retries only on 429/5xx/transport errors (4xx are permanent), and messages are truncated on rune boundaries (4096 limit).
• Alert channels are stored in the database and editable from Settings (Telegram, ntfy, Gotify, syslog, SMTP email, webhook), effective without restarting the service. Env vars only seed the first run.
• Settings UI: new "Alert channels" panel with per-channel status, inline configuration and a test button. Secrets are write-only (never returned by the API, only a token_set flag).
• New endpoints: GET /api/channels, PUT/DELETE /api/channels/{name}, POST /api/channels/{name}/test.

Notes:

• Webhook has no test button: delivery is asynchronous (queue + DLQ), there is no synchronous result to show.
• Push (Web Push) stays informational; VAPID keys are generated by the installer.

Tests: full Go suite green, go vet clean. Playwright UI smoke 15/15 (configure Telegram/ntfy/email/webhook, hot save, secrets not exposed, real delivery, disable).

closes #134