Lock sharp and source-map-js at their patched releases

The dependency audit fails since two advisories were published against
packages that Next.js brings in:

- sharp before 0.35.5, a vulnerability in the librsvg it bundles
  (GHSA-wq5f-xc86-pv6w);
- source-map-js before 1.2.2, a denial of service through indexed
  source-map sections (GHSA-68fv-2mgg-jv7q).

Next.js asks for sharp ^0.35.4 and PostCSS for source-map-js ^1.2.1, so the
patched releases were already allowed and only the lockfile held the old
ones. The lockfile now resolves sharp 0.35.5, with its platform packages,
and source-map-js 1.2.2. No other package changes, and no override or
ignored advisory is added.

The account API kept everything in the memory of its process, so every
restart and every deployment of the API deleted all accounts and signed
everyone out.

PostgresAccountStore implements IAccountStore on PostgreSQL through Npgsql.
The three uniqueness rules are unique constraints, so two requests that race
for one name, address or Google identity are decided by the database. A
refused account is an ordinary answer, not an exception, and the rules are
reported in the order the in-memory store reports them. A session refers to
its account, and a session of an account that is not stored is refused; the
in-memory store now refuses it as well.

The schema is a list of numbered steps. The service takes the missing ones
when it starts, in one transaction under an advisory lock, and does not
start when the database cannot be reached or is newer than the release.
/health answers 503 while the database does not answer.

ConnectionStrings:Accounts selects PostgreSQL. A production service that is
given no database does not start; keeping accounts in memory there has to
be asked for with Accounts:Store=Memory. Development and the tests keep the
in-memory store by default.

The store gains RemoveExpiredSessionsAsync, and a background service calls
it at the start and once an hour: a session whose token is never presented
again no longer keeps its row for ever.

PostgreSQL keeps a timestamp to the microsecond. The store cuts a moment to
that precision and returns what it stored, and the service uses the
returned account, so the creation time in the registration response equals
the one every later read returns.

Tests. AccountStoreContractTests states the rules once and runs them on
both stores. Further tests cover the schema steps applied concurrently and a
database that is too new, the choice of the store from the configuration,
the removal of expired sessions, and an account with its session surviving
a restart of the service over HTTP. The PostgreSQL tests read their
database from PROGMAWEB_TEST_POSTGRES and work in a schema of their own
each. Without the variable they are skipped; CI starts PostgreSQL 16 for
them, and a test fails the run when the variable is missing there.

Operations. ops/README.md describes the database: the Unix socket with peer
authentication, so that no password exists, the one-time preparation, and
rollback across a schema step. The service unit starts after PostgreSQL.
docs/LIMITATIONS.md no longer lists the in-memory store and now lists the
missing backup and the missing deletion of accounts.

Run the account API without a Google client

Require documentation for every item of the API reference

With an empty Google client identifier or secret the API answered every
request with 500, including registration, password sign-in and the status
route. The Google scheme was always registered, and the authentication
middleware initializes every remote scheme for each request; with empty
credentials the validation of the Google options threw there, before any
endpoint ran. The API could not be started locally without credentials, and
the "Google sign-in is unavailable" answer of the start endpoint was never
reached.

The Google scheme is now registered only when both values are configured.
Without it the service runs normally, the start endpoint answers 503, the
completion endpoint redirects to the failed sign-in page without a session,
and the callback path is not a route. With both values nothing changes.

The test factory passes the two values as host settings. The program makes
the decision while it registers services, and configuration sources a test
factory adds are applied only when the host is built, after that point.

New tests run the service without a Google client: the ordinary routes,
a full register, sign-out and sign-in round trip, and the three Google
paths. They failed with 500 against the previous program and pass now, also
when the environment of the test run carries Google credentials.

The guides no longer list this as a limitation or ask for placeholder
credentials.

Deploying this change restarts the API, which deletes every account while
the store is in memory.

The guides merged from main still said that the DocFX gate does not require
documentation comments. With this branch it does, so:

- docs/DEVELOPMENT.md and README.md list the coverage check next to the
  DocFX command and say what fails the gate.
- docs/LIMITATIONS.md no longer lists missing documentation comments as a
  limitation.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>