Soft-delete streak goals and fix backdated start date bug

[anscg]

Summary

• Streak goals are now soft-deleted (via Discardable) instead of permanently destroyed when a user sets a new goal or their streak breaks — preserving history
• Replaced the unique index on user_id with a partial index scoped to kept (non-discarded) records, allowing multiple historical goals per user
• user.streak_goal association now scoped to .kept so it always returns the active goal
• Fixed a bug where new goals were backdated to the start of the user's current streak, causing them to be immediately completable — new goals now always start from today

Root cause of the double-announcement bug (user 252)

Pegoku completed their 7-day goal on Apr 20, then immediately set a new 3-day goal. Because started_on was backdated to the streak start (Apr 14), the 3-day goal was already satisfied on creation. The next day when reconcile_missed_days ran a freeze and called check_goal_completion, it fired the announcement immediately.