Coop: per-gift voting timers (6h) — gifts resolve throughout the day

Replace the single daily-tick gift resolution with independent per-gift
expiries. Each gift now has its own 6h voting window; once that elapses,
the votes are tallied, the sender gets DM feedback, and the live game-room
post is edited to reveal the type and resolved modifier. The modifier
sits on the run waiting for the next floor resolution to merge it in.

Effect:
- Gifts fire continuously throughout the day rather than all at once at
  08:00 UTC, surfacing sender activity in real time.
- Senders get fast feedback (~6h instead of waiting for the next daily
  tick).
- Floor resolution becomes purely "sum pending modifiers from already-
  resolved gifts" — cleaner separation of concerns.
- Atomic per-gift apply via markCoopGiftApplied (UPDATE...WHERE applied_at
  IS NULL) prevents double-application on retry.

Schema additions:
- coop_dungeon_gifts.expires_at (when voting closes)
- coop_dungeon_gifts.applied_at (when modifier merged into a floor)
- Migration entries provided.

Defensive backstop: floor resolution still force-resolves any gifts whose
expiry was missed (e.g., bot down during the timer window).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/db/db.go

@@ -138,6 +138,8 @@ func runMigrations(d *sql.DB) error {
 		`ALTER TABLE coop_dungeon_runs ADD COLUMN last_resolved_day INTEGER NOT NULL DEFAULT 0`,
 		`ALTER TABLE coop_dungeon_members ADD COLUMN member_payout INTEGER`,
 		`ALTER TABLE coop_dungeon_runs ADD COLUMN invite_post_id TEXT NOT NULL DEFAULT ''`,
+		`ALTER TABLE coop_dungeon_gifts ADD COLUMN expires_at DATETIME`,
+		`ALTER TABLE coop_dungeon_gifts ADD COLUMN applied_at DATETIME`,
 	}
 	for _, stmt := range columnMigrations {
 		if _, err := d.Exec(stmt); err != nil {
@@ -1445,7 +1447,9 @@ CREATE TABLE IF NOT EXISTS coop_dungeon_gifts (
     modifier      INTEGER NOT NULL DEFAULT 0,
     post_event_id TEXT,
     sent_at       DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
-    resolved_at   DATETIME
+    expires_at    DATETIME,                      -- voting closes; tally fires when reached
+    resolved_at   DATETIME,                      -- vote tallied (vote_result/modifier set)
+    applied_at    DATETIME                       -- modifier merged into a floor success roll
 );
 CREATE INDEX IF NOT EXISTS idx_coop_gifts_run_day ON coop_dungeon_gifts(run_id, day, vote_result);
 `