[TrueFurina]

Summary

Improves .env.example readability for first-time installers per the task requirements:

• Adds a short explanation for each configurable value
• Clarifies options whose purpose is not obvious from the variable name (e.g. DATABASE_HOST, TP_TLS_*, mail settings)
• Marks every default as [dev] (development/testing) or [prod] (change for production), e.g. DATABASE_PASSWORD default is explicitly called out as local-testing-only with an openssl rand command for production
• Groups related variables under section headers

Changes

• .env.example: restructured comments, added per-variable docs, [dev]/[prod] markers, kept all variable names and values identical

Acceptance criteria

• Short explanation for each configurable value added
• Purpose clarified where not obvious from the name
• Defaults marked as dev vs production intent
• No variable names or values changed (comment-only diff)

[TrueFurina]

两个红项都不是本改动引起的，逐条说明（供维护者判断）。

1. pytest：测试全过，红在「向 PR 发评论」这一步

=========== 3338 passed, 12 skipped, 1 warning in 454.18s (0:07:34) ===========
##[error]Resource not accessible by integration - https://docs.github.com/rest/issues/comments#create-an-issue-comment

测试本身 3338 全通过。失败发生在其后的发评论步骤：本 PR 来自 fork，
pull_request 事件下默认 GITHUB_TOKEN 是只读，无法创建 issue comment。

旁证（同仓库其他 PR）：

PR	来源	pytest
#974 / #973 / #972 / #971	本仓库分支（token 可写）	✅ pass
#901（本 PR） / #975	fork	❌ 同一 Resource not accessible

建议修法（任选其一）：

- name: Comment PR
  if: github.event.pull_request.head.repo.full_name == github.repository

或对该 step 加 continue-on-error: true。不建议为此改用 pull_request_target
（那会让 fork 代码获得写权限，风险更大）。

2. codecov-upload：动作已弃用

This action is being deprecated in favor of 'codecov-action'.
Please update CI accordingly to use 'codecov-action@v5' with 'report_type: test_results'.

3. 本 PR 当前状态

mergeable=false、state=dirty —— 与 main 存在文本冲突，需要 rebase 后才能合并。
这是 docs/ 下的文档改动（.env.example 逐变量说明），冲突解决无技术风险。
需要的话我可以把它重新基于当前 main 整理干净，只是不想未经确认就改写已推送的分支。