Finish M2: run lock, systemd units, discover; drop the web UI

Designing the configuration UI in full made the case against building it. Its
audience would be people who find TOML hard, but with bring-your-own OAuth
client settled, every user must first create a Google Cloud project, configure
a consent screen and put a secret in a keyring — a far higher bar than editing
thirty lines of config. Anyone who clears it can edit the file; anyone who
cannot never reaches the file. Against that stood three dependencies, five
modules, an auth.rs refactor and a security surface guarding something that
reads the config and touches the keyring. `doctor` and `status` had already
absorbed most of what it was for. The reasoning is recorded in TODO.md and
SPECS.md rather than left as an apparent oversight.

The run lock is not a UI feature and closes a gap that already existed: nothing
stopped a timer firing into a hand-run cycle, and two cycles interleaving
writes over the same vdirs is what the design otherwise avoids. flock is used
rather than a pid file because the kernel releases it however the process ends,
so a crash cannot leave a lock to clear by hand — which also means a lock we
failed to take is held by a live process, so the pid in it is worth reporting.

The one idea worth keeping from the UI design was collection discovery, which
needed no web layer. `calcalist discover` prints a ready-to-paste endpoint
block per calendar a server offers, removing the most error-prone field in the
config. pimsync's discovery output is undocumented, so the format was
established against a real server first. Two things it teaches: everything
arrives on stdout including failures, and a pair has two storages, so pimsync
reports the scratch vdir's contents too — parsing anchors on the heading naming
the server, or a probe directory's leftovers would be offered as the user's
calendars.

Verified against Posteo as well as Radicale: all four calendars found, the
first matching the URL already configured.

Also fixes a real defect in the test harness rather than its symptom. Ports were
chosen by binding one and letting go, so two tests could pick the same number —
and the loser's readiness check then succeeded against the winner's server,
silently sharing it. Startup now confirms the child we spawned is the one alive,
retries on another port if not, and waits for a real HTTP response rather than
an open socket.

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

SPECS.md

@@ -4,7 +4,9 @@
 
 Small Rust based commandline tool with utility systemd service file that can aggregate and sync events between CalDAV/Google Calendar/iCal. Users who have a Google Calendar can create a new calendar that syncs events from several CalDAV calendars. CalDAV users can sync several Google Calendars into one calendar. This should be doable both ways, so the aggregating calendar needs to be able to distinguish where its events came from and also have a way to create new events that get synced to the correct source. In addition iCal feeds can also be aggregated but they remain one way only.
 
-Configuration via a simple web interface. Configuration is saved in a portable toml file that can be easily migrated.
+Configuration is saved in a portable toml file that can be easily migrated.
+
+The web interface this originally called for was designed and then dropped. Setting up Google's OAuth client — a Cloud project, a consent screen, a secret in a keyring — is a far higher bar than editing thirty lines of TOML, and no interface can remove it, so a configuration UI would have served an audience that never reaches the configuration. `calcalist doctor` reports every problem in one pass, `calcalist status` names the routing markers an event can carry, and `calcalist discover` prints ready-to-paste endpoint blocks for a server's calendars — which is what the interface was actually for.
 
 ## Architecture