Store a goal against the period it was scored in, like a substitution

[JaspervdM80]

What is wrong

The two kinds of match event are stored differently, and only one of them records where in the match it happened.

	GameSubstitution	GameGoal
Which half	GamePeriodId → period → Half()	nothing
When	AtSeconds (real elapsed clock)	nothing
Minute	derived at display time	Minute + AdditionalMinute, frozen when it was written

A goal's half is only implied. Minute is capped at the end of the half it was scored in, so on a 70-minute game 1–35 reads as the first half and 36–70 as the second. That is inference from a display value, not a fact on the row, and it has two consequences:

• It moves under stored data. Editing GameDurationMinutes shifts the half boundary, and goals already on file are silently reinterpreted — a goal written 36 stops meaning what it meant.
• The minute cannot be corrected. A substitution's minute is derived from AtSeconds and its period, so fixing a period's StartedAtSeconds fixes the substitutions with it. A goal's minute is frozen, so the same correction leaves the goals wrong and the two kinds of event disagreeing on the same timeline.

AdditionalMinute and the pair-comparison ordering that goes with it (MatchMinute.CompareTo) exist only because the goal row has no real clock reading to sort on.

What to do instead

Give a goal the same two facts a substitution already carries:

• GamePeriodId — nullable FK to GamePeriod, set when the goal is logged from /live
• AtSeconds — nullable, the real elapsed match clock, same source as GameSubstitution.AtSeconds

Both nullable because a goal typed by hand on /result has neither a clock nor a period behind it. Minute stays as the fallback for exactly those rows, rather than being presentation stored on every goal.

Then:

• the displayed minute derives through MatchClockReport for both event types, from one code path
• ordering is by real elapsed seconds instead of comparing a (minute, additional) pair
• GameGoal.AdditionalMinute and MatchMinute.CompareTo can go
• correcting a period's timings corrects the goals in it, as it already does for substitutions

Scope

• Migration: add GamePeriodId (nullable FK, cascade like GameSubstitution) and AtSeconds (nullable int) to GameGoals; drop AdditionalMinute. No backfill — nothing left in an old row says whether a stored 37 was stoppage time or a minute typed in by hand, so existing goals keep reading and sorting exactly as they do today, off Minute alone.
• MatchGoalService.LogGoalAsync — write the period and the clock reading instead of the minute pair
• MatchClockReport — one MinuteOf path covering goals and substitutions, plus the fallback for rows with no clock
• ScoreProgressionReport, LiveMatch.Timeline, MatchResult goal list — order on elapsed seconds, falling back to Minute for rows without one
• Docs: models.md (the GameGoal table and the note under GameSubstitution), ui_components.md (the MatchMinute bullet), known_issues.md (the "a goal's minute is not one number" entry describes the shape this replaces)

Background

The 35+2 display and the pair ordering landed in "Run the live match in halves, and write a minute the way football does" (#80 follow-up branch). That fixed the visible bug — first-half stoppage events sorting after the restart — but stored the fix as a display value on the goal rather than recording where the goal actually happened. This issue is the model change that makes the same behaviour fall out of the data.