chore: release 1.1.0

[lorem-dev]

Doppel 1.1.0. Templates can see what Doppel knows, and the two credentials can
come from the environment.

System variables

Nine, bound into every template Doppel renders, whether or not the mock asked for
anything: proxy_name, mock_name, doppel_version, request_id, method,
path, host, peer_ip, real_ip. Before this, the things every mock wants --
which proxy answered, which version, who called -- had to be extracted by hand in
each one, and two of them could not be extracted at all.

real_ip is the chain a proxy in front writes: X-Real-IP, then the leftmost
X-Forwarded-For entry, then the connection's own address, so it always has a
value. peer_ip beside it is deliberate: one is a claim anybody can make and the
other is a fact.

They are reserved -- bound after the operator's extractions, so a template always
means what the documentation says -- and a mock that extracts into one of those
names is called out at startup, because the extraction still costs a header read
and its result is then thrown away.

An address that can depend on the request

server.external_url may be a template over those variables:

server:
  external_url: "http://{{ host }}/"
  external_url: "https://{{ proxy_name }}.gw.example.com/"

Opt-in for a reason, and the documentation says so in a warning: Host arrives
from the client, so a deployment that builds a redirect out of it is choosing to
let a caller decide where its own redirects point. A template that renders to
something unusable means no rewriting for that request rather than a failed one.

Credentials from the environment

DOPPEL_SENTRY_DSN provides the Sentry DSN or overrides sentry.dsn, the way
DOPPEL_ADMIN_TOKENS already does for tokens: a DSN carries the key that
authorises sending events, and a deployment that keeps credentials in the
environment should not have to write one into a document the admin API returns.
Startup logs which source won. An empty variable counts as unset, deliberately --
a failed interpolation leaves a configured DSN in force rather than silently
switching error reporting off.

Not breaking

Variable names in main.example.yaml and the documentation are snake_case now,
and an operator's own names are untouched: Jinja accepts either. Nothing else
changed shape, which is why this is a minor bump.

Verification

make gate clean with the PostgreSQL suites included -- 817 Rust tests, 127 jest,
eslint, both TypeScript projects, the size budgets, a strict docs build, the
schema, the parameter reference, the documentation links and the licence check --
and make e2e clean, 81 Playwright tests. release_notes.py 1.1.0 composes a body
from the section this branch writes.

The pre-release pass found one thing, fixed in 52a008e before the release commit:
DOCKERHUB.md had fallen behind on the environment variables, and writing that
section turned up a claim of mine that was nearly false -- nothing that ships
enables the sentry feature, so a reader following it would have set a variable
that does nothing. Both pages say so now.

After the merge

git checkout main && git pull
git tag -a v1.1.0 -m "doppel 1.1.0"
git push origin v1.1.0