Coop: stack same-type gifts on the same day into a single vote post

Multiple baskets (or multiple mimics) sent during the same day now share
one game-room post, one vote, one resolution. First-in becomes the stack
"lead"; subsequent same-type sends become followers that inherit the
lead's deadline and votes.

Behavior:
- Send a basket → new lead, new post, 6h timer starts
- Send another basket within the window → silently joins the stack,
  edits lead's post to bump count
- At stack size 2, TwinBee adds a "this gift looks REALLY special" line
  (one of 5 variants picked deterministically by lead id). No further
  escalation as the stack grows
- One !coop giftvote on the lead's id covers the whole stack
- Resolution applies the same outcome to every gift in the stack;
  modifier is N × ±6
- End-of-run gift log groups by stack with all senders attributed

Schema: coop_dungeon_gifts.stack_lead_id INTEGER (NULL for lead/standalone,
otherwise points at lead's id). Migration entry included.

Why first-in deadline (vs extending on each follower): exploitability.
Saboteurs could spam-extend a stack to delay resolution. Locked deadline
keeps senders honest about timing.

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

internal/db/db.go

@@ -140,6 +140,7 @@ func runMigrations(d *sql.DB) error {
 		`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`,
+		`ALTER TABLE coop_dungeon_gifts ADD COLUMN stack_lead_id INTEGER`,
 	}
 	for _, stmt := range columnMigrations {
 		if _, err := d.Exec(stmt); err != nil {
@@ -1449,7 +1450,8 @@ CREATE TABLE IF NOT EXISTS coop_dungeon_gifts (
     sent_at       DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
     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
+    applied_at    DATETIME,                      -- modifier merged into a floor success roll
+    stack_lead_id INTEGER                        -- NULL for lead/standalone; follower points to lead's id
 );
 CREATE INDEX IF NOT EXISTS idx_coop_gifts_run_day ON coop_dungeon_gifts(run_id, day, vote_result);
 `