test: the PostgreSQL path only ever runs in CI

[L4XB]

Problem

Running the API suite locally ends with:

SKIPPED [1] tests/test_postgres_runtime.py:27: requires the dedicated CI PostgreSQL service
2901 passed, 1 skipped

Every contributor's local run skips it. The deployment runs PostgreSQL; the
local suite runs SQLite. So the one test that exercises the database the product
actually uses is the one nobody sees fail until CI does — usually after a push,
on someone else's branch.

What to do

Give contributors a way to run it without hand-assembling a database. A
make test-pg (or a documented docker compose run) that starts a throwaway
PostgreSQL, runs that file against it, and removes the container afterwards.

Two things it must not do:

• Leave a container or volume behind on failure
• Require the full self-hosting stack — this is one database for one test file,
  not a deployment

Skipping must still work when Docker is unavailable. A contributor without it
should get the same skip they get today, not an error.

Where

• services/api/tests/test_postgres_runtime.py — the skip condition at line 27
• Makefile
• CONTRIBUTING.md — the API section under "Local setup and checks"

Acceptance criteria

• One command runs the PostgreSQL test from a clean checkout
• Nothing is left running or stored afterwards, including after a failure
• Without Docker, the test still skips rather than erroring
• CONTRIBUTING.md names the command