fix(packaging): the hub unit can start, and nothing carries the migration path

`ExecStartPre` ran `alembic -c /opt/meshbay-hub/migrations/alembic.ini upgrade
head`. The build does stage that file, so the path existed and the contents
were wrong: `alembic.ini` resolves `script_location` with `%(here)s`, so the
copy pointed at `/opt/meshbay-hub/migrations/src/meshbay_hub/db/migrations` —
which nothing installs, because the migrations ship inside `meshbay_hub`, in
the shared venv.

`ExecStartPre` failing stops the unit. A hub installed from the RPM or the DEB
could not start at all, and nothing noticed because the one live deployment
was assembled by hand — the same shape as the node unit that carried `User=`
into the user unit directory.

The same `%(here)s` trap was already found once on the server, where a stray
`alembic.ini` resolved to a month-old snapshot of the tree. Twice is a trap
rather than an accident, so the fix is that the path is no longer written down
anywhere: `meshbay-hub migrate` asks the installed package where its own
migrations are, which is correct for the RPM, the DEB, a venv and a checkout.
The build stages no `alembic.ini`; the repo keeps its own for `alembic
revision` and for deploy scripts that already work.

`env.py` now prefers a URL the caller resolved over re-reading the environment
itself, so `migrate --config` connects with exactly the string the server
will — one resolution, not two that agree until they do not.

Six tests, three of which fail against the unit as it was. They read the
directives rather than the file, because searching the whole thing finds the
comment explaining a directive and calls that the directive.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T4YmK41VsEURWFdop4EEeT

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