feat: plan prices, payment toggles and HWID limit now editable live from the admin panel, no restart

Closes the last "still .env-only" gap from the backlog (tariffs/HWID) and
fixes a real bug found while building it: payment provider credentials and
enabled-flags were frozen in api.py's process at import time, so a Platega
secret rotation via the settings UI would leave api.py verifying inbound
webhooks against the OLD secret until a manual `mbs restart` — while
bot.py (which does get restarted on save) already had the new one. Same
class of staleness affected HWID_LIMIT_ENABLED/HWID_FALLBACK_LIMIT and
plan prices, neither of which had any settings UI at all before this.

New `settings.py` module: get_plans()/get_plans_by_code() (live prices,
falls back to config.py defaults), get_payment_settings(), get_hwid_settings(),
yookassa_credentials()/platega_credentials(), set_plan_prices() — all backed
by a new batched legal.read_env_vars() (one file read for N keys instead of
N reads) and legal.update_env_var() (moved out of api.py's private
_update_env_var, which is now a one-line delegate to avoid duplicating the
same env-file-rewrite logic in two places).

api.py and bot.py no longer import PLANS/PLANS_BY_CODE/PAYMENTS_ENABLED/
HWID_LIMIT_ENABLED/HWID_FALLBACK_LIMIT from config as frozen constants —
every read goes through settings.py instead. payments.py no longer imports
YOOKASSA_*/PLATEGA_* from config either; every provider call (create/check
payment, verify webhook signature) reads live credentials at call time.
Every call site inside a loop hoists the live lookup before the loop first
(same N+1 discipline as the rest of tonight), so this doesn't regress
get_subscription's hot path — one settings.get_hwid_settings() call per
request, same as before.

New routes: GET/POST /admin/api/payments/plan-settings (per-plan prices +
a payments_enabled master toggle — there was previously no way to turn
payment collection back off without deleting provider credentials),
GET/POST /admin/api/hwid-settings. Both validate input strictly (prices:
non-negative int; HWID limit: 1-1000) and reject the whole request instead
of partially applying on bad input.

admin.html: new "Тарифы" section in Платежи (price inputs rendered from
the live plan list + payments toggle) and "Лимит устройств (HWID)" in
Настройки, both using the existing .check checkbox / plain-input styling
(no native <select>, per the earlier white-popup complaint). Removed the
now-incorrect "выполни mbs restart" copy from the YooKassa/Platega settings
hints and save-result messages, and added a doc-block for HWID (never had
one) plus extended the Платежи doc-block to mention live-apply. Also
dropped a dead `import links` in bot.py caught by pyflakes while verifying
this.

Verification: api.py/bot.py still can't be imported on this Windows
machine (no prebuilt pydantic-core wheel for Python 3.14, confirmed again
by a fresh pip attempt — same wall as every prior session), so relied on
what's actually exercisable: py_compile + pyflakes (zero undefined names)
across every module including api.py/bot.py, a real runtime test against
an isolated .env fixture covering live price/toggle/HWID reads with zero
reimport, write-idempotency (no duplicate .env lines on repeated saves),
and the concrete bug this fixes end to end — computed an HMAC signature
against an old Platega secret, rotated the secret via update_env_var (the
same call the settings route makes), confirmed the old signature is now
rejected and a new one computed against the rotated secret verifies, all
in the same process with no reimport. Also ran the new CI step's exact
heredoc locally byte-for-byte before adding it to ci.yml. GitHub Actions
still won't trigger for this account (still under abuse-review, ticket
open >2 days) so this is the same substitute-for-CI rigor used all night.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.env.example

@@ -34,6 +34,8 @@ DE1_ADDRESS=de1.example.com
 
 # Payments — off by default, bot keeps handing out free subscriptions on button press.
 # Flip to true only once at least one provider below is configured and its webhook is live.
+# All of this (toggle, prices, provider keys) is also editable live from the admin panel
+# (Платежи tab) after first boot — no need to hand-edit this file or restart afterwards.
 PAYMENTS_ENABLED=false
 
 # Prices in RUB per plan (whole numbers). Only used when PAYMENTS_ENABLED=true.
@@ -58,6 +60,6 @@ PLATEGA_SECRET=
 # Device limit (HWID) — off by default. Requires the VPN client app to send an
 # x-hwid header on subscription fetch (Happ/v2rayTun-class apps do this); clients
 # that don't send it get refused once enabled, so only flip this on if your users'
-# apps actually support it.
+# apps actually support it. Also editable live from Настройки in the admin panel.
 HWID_LIMIT_ENABLED=false
 HWID_FALLBACK_LIMIT=3