Decide request TLS in one place, per request (closes #269)

Two places decided whether a request was TLS, by two different means,
and they disagreed.

The session cookie's Secure attribute was fixed at startup from
!Config.IsDev(). "dev" is the environment when WEBHOOKER_ENVIRONMENT is
unset, so a deployment terminating TLS at a proxy without also setting
the environment shipped the authentication cookie with no Secure
attribute -- on the same response as a CSRF cookie that had one. It
failed silently: everything kept working, so nothing prompted anyone to
look.

The CSRF middleware's per-request check compared X-Forwarded-Proto with
== "https" exactly, so "HTTPS", "https, http" and "https,https" all took
the plaintext path. Uppercase is legal for a case-insensitive token and
the comma forms are what a proxy chained behind another proxy emits by
appending rather than replacing. On that path gorilla/csrf stops
enforcing the strict Referer check on a site that genuinely is HTTPS.

Both now go through internal/reqtls.IsTLS, which folds case and takes
the leftmost comma-separated element -- the hop nearest the client, and
so the one a cookie's Secure attribute is about. A third package is
needed because internal/middleware already imports internal/session, so
session cannot import middleware back.

Per-request beat a startup warning for the session cookie because it
turned out to need no restructuring: gorilla/sessions gives every
session its own copy of the store's Options and renders the cookie from
that copy, and every session-cookie write here already goes through
Session.Save or Session.Regenerate, both of which hold the request. The
store's template Secure becomes true so that a write path added later
which forgets to track the transport fails visibly instead of silently
dropping Secure.

The flag tracks the transport in both directions rather than latching
on. Secure over plaintext is discarded by the browser without an error,
which would make a plain-HTTP local run impossible to log into -- and
would also void the deletion cookies in Destroy and Regenerate, leaving
a session the user just tried to end still live.

A third site that makes this decision, internal/handlers'
BaseURL construction, assigns the raw header straight into the URL
scheme. It is left alone here and filed separately.

README.md

@@ -74,7 +74,7 @@ you can place variables in a `.env` file in the project root (loaded
 automatically via `godotenv/autoload`).
 
 The environment is selected by setting `WEBHOOKER_ENVIRONMENT` to `dev`
-or `prod` (default: `dev`). The setting controls several behaviors:
+or `prod` (default: `dev`). The setting controls exactly one behavior:
 
 | Behavior | `dev`                   | `prod`           |
 | -------- | ----------------------- | ---------------- |
@@ -104,7 +104,11 @@ direct TLS, or over plain HTTP during development — a plain-HTTP local
 run gets non-`Secure` cookies and remains usable, and a proxied
 deployment gets `Secure` ones without the operator setting anything.
 When running behind a reverse proxy, ensure it sets the
-`X-Forwarded-Proto: https` header.
+`X-Forwarded-Proto: https` header. Unlike `X-Forwarded-For`, this
+header is read from any peer and is **not** gated by
+`TRUSTED_PROXIES`; a correctly configured proxy overwrites whatever a
+client sent. On a listener exposed directly to clients, any client can
+assert it, so do not run one without a proxy in front.
 
 All other differences (log format, security headers, etc.) are
 independent of the environment setting — log format is determined by