Enforce the Devii task quotas with atomic reservations

The creation and run quotas were checked and then acted on, so two concurrent
create_task calls or two schedulers could both pass the check and overshoot the
limit. Both are now a single conditional INSERT decided on the driver rowcount:
reserve_run takes a run slot after the claim and releases the claim by deferring
when the quota is spent, and insert_task_within_quota does the same for the task
row itself. Racing twelve and sixteen processes now yields exactly the limit.

The atomic insert names its columns, and dataset skips a None valued key when it
creates a table lazily, so the store declares the full task column set up front.
Both the column and index ensures now tolerate a concurrent duplicate, since
several processes build a store at once and SQLite DDL is not idempotent.

Adds the quota, task-run context, guard, store and scheduler test suites, and
documents the chokepoints and the unhackable task-run flag.

tests/unit/services/devii/tasks/limits.py

@@ -7,6 +7,8 @@ from devplacepy.services.devii.tasks import limits
 from devplacepy.services.devii.tasks.schedule import now_utc, to_iso
 from devplacepy.utils import generate_uid
 
+SIGNUP_AFTER_PRIMARY_ADMIN = "2099-01-01T00:00:00"
+
 
 def _account(local_db, role):
     uid = generate_uid()
@@ -16,7 +18,7 @@ def _account(local_db, role):
             "username": f"lim-{uid[-10:]}",
             "role": role,
             "deleted_at": None,
-            "created_at": to_iso(now_utc()),
+            "created_at": SIGNUP_AFTER_PRIMARY_ADMIN,
         }
     )
     invalidate_admins_cache()