chore: consolidate per-app env files into a single .env + runtime host injection

[dzhhem]

Description

Collapses the env-template sprawl into one .env + one .env.example per app.
Previously each app shipped a local example plus a near-duplicate .docker.example
that differed only in host names — every new variable had to be added in two or three
files and kept in sync by hand.

The single template now targets local (non-Docker) hosts. Docker hosts are injected
at runtime instead of being duplicated:

• services — Compose environment: overrides (compose.dev.yaml); prod adds
  NODE_ENV + build args (compose.yaml). web: NEXT_PUBLIC_API_URL,
  NEXT_PUBLIC_TELEGRAM_BOT_ID. bot: API_URL → api, REDIS_URL → redis.
• api tests — new scripts/test-env.cjs rewrites DB/Redis/Mongo hosts to the
  Docker service names for :dx test scripts.

Every .env.example is also split into banner-delimited REQUIRED (app won't boot
without these) and OPTIONAL (safe defaults + feature toggles) blocks, so required
vs optional is obvious at a glance. Docs updated to match.

Deleted: apps/*/.env*.docker.example (api, api test, web, bot).
Real .env / .env.docker remain gitignored — only .example files are committed.

Closes #98

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)

How Has This Been Tested?

• Unit tests (Jest/Vitest)

• Integration tests

• Manual testing (screenshots/screencasts encouraged)

• ran bash dx run test:api:light:dx — host rewrite via scripts/test-env.cjs
  resolves postgres/redis correctly; suite green

• brought up dev stack (bash dx dev) — api, web, bot connect using Compose
  environment: host overrides from the single .env

• verified no variables were dropped vs the old templates and that example/test diffs are intended functional lines only

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have documented non-obvious behavior or constraints where necessary
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• (If API) Database migrations have been created and tested
• (If UI) Changes look good on mobile and desktop