feat(analytics): optional bot filtering via isbot (SKIP_BOTS=false by default)

[codedsultan]

What
Add an opt-in flag to skip inserting analytics rows for obvious bots. Uses isbot to detect common crawlers.

Default
SKIP_BOTS=false (no behaviour change).

How

• Add SKIP_BOTS to central env (server/env.js) and .example.env.
• In the visit logging path, if SKIP_BOTS=true and UA is a bot, only increment the link hit counter (no visit row).

Normal user traffic unchanged.

Why
Reduces analytics noise without affecting redirects or link counters.

Note: This PR branch off PR #937 (ua-parser swap).

[poeti8]

I don't want to increment visits count if it's a visit by a bot. It's already done in links.handler.js:533.