fix: auto initialize default send balance (#985)

* fix: auto initialize default send balance

* fix: tighten send access auto init flow

* refactor: centralize send balance state

* fix: separate legacy repair from admin control in send balance

Add an `address_sender.source` column to distinguish legacy / auto /
user / admin rows. `ensureDefaultSendBalance` now only repairs rows
with `source IS NULL`, so admin-disabled and user-requested rows are
never overwritten. Admin POST writes tag `source = 'admin'`; new
auto-init inserts tag `'auto'`; `requestSendMailAccess` inserts tag
`'user'`.

Bumps DB_VERSION to v0.0.8 with the usual `PRAGMA table_info` guarded
ALTER, plus a standalone SQL patch under db/.

Adds E2E regressions: legacy repair path, admin-disabled rows stay
disabled across settings and send, send after admin deletion
auto-initializes a fresh row.

* fix: drop runtime legacy repair; backfill source='legacy' on migrate

Pre-v0.0.8 schema cannot distinguish legacy request-send-access
remnants from admin-disabled rows — both share `balance = 0,
enabled = 0`. Letting ensureDefaultSendBalance repair that shape on
upgrade could silently re-enable an admin-disabled row.

Remove the runtime repair path entirely:

- `ensureDefaultSendBalance` now uses `ON CONFLICT(address) DO NOTHING`;
  existing rows are never touched.
- The v0.0.8 migration (and the matching SQL patch) backfills every
  pre-existing row with `source = 'legacy'`, making pre-migration
  state explicitly off-limits to runtime auto-init.
- E2E: flip the legacy test to the negative direction — a
  `source='legacy'` zero-balance row stays untouched by settings
  reads and send attempts. Harden `resetSenderToLegacy` to return
  404 when `meta.changes < 1`.
- Update changelog and docs: legacy/admin-disabled rows must be
  restored manually via the admin UI.

* refactor: collapse send balance auto-init to missing-row insert

Per review feedback: the runtime guarantee we actually need is
"create an address_sender row when one is missing, leave existing
rows alone". Once `ensureDefaultSendBalance` switched to
`ON CONFLICT DO NOTHING`, the `source` column, the v0.0.8 migration,
and the `resetSenderToLegacy` test endpoint became dead weight —
the DO NOTHING path already protects admin-disabled and admin-edited
rows without any provenance metadata.

- Drop `address_sender.source` and the v0.0.8 migration; revert
  DB_VERSION to v0.0.7. No schema change ships with this PR.
- Strip the `source` field from `ensureDefaultSendBalance`,
  `requestSendMailAccess`, and the admin-update path.
- Remove the `/admin/test/reset_sender_to_legacy` test endpoint and
  its E2E helper; the negative legacy-repair test it served is no
  longer needed because the runtime no longer touches existing rows.
- E2E coverage stays focused on the three guardrails: missing-row
  auto-init, admin-disabled rows stay disabled, admin deletion
  triggers a fresh re-insert.
- Tighten changelog and docs to "auto-initialize missing rows".

* docs: align common-issues with missing-row-only auto-init

The FAQ entries for "DEFAULT_SEND_BALANCE set but still No balance"
still described the old behaviour of repairing legacy
`balance = 0 && enabled = 0` rows. Rewrite both zh and en rows to
match the current runtime: only addresses with no existing
`address_sender` row get auto-initialised; legacy, admin-disabled,
and admin-edited rows must be restored manually through the admin
console.

frontend/src/views/index/SendMail.vue

@@ -33,7 +33,7 @@ const { t } = useI18n({
             toMailEmpty: 'Recipient address is empty',
             contentEmpty: 'Content is empty',
             requestAccess: 'Request Access',
-            requestAccessTip: 'You need to request access to send mail, if have request, please contact admin.',
+            requestAccessTip: 'No send balance yet. If your admin enabled a default balance it should be assigned automatically; otherwise request access or contact the admin.',
             send_balance: 'Send Mail Balance Left',
             text: 'Text',
             html: 'HTML',
@@ -54,7 +54,7 @@ const { t } = useI18n({
             toMailEmpty: '收件人地址不能为空',
             contentEmpty: '内容不能为空',
             requestAccess: '申请权限',
-            requestAccessTip: '您需要申请权限才能发送邮件, 如果已经申请过, 请联系管理员提升额度。',
+            requestAccessTip: '当前还没有可用的发信额度。如果管理员启用了默认额度，会自动发放；否则请申请权限或联系管理员处理。',
             send_balance: '剩余发送邮件额度',
             text: '文本',
             html: 'HTML',