Adv 2.0 L5 close-out: loader rewire + adventure_characters purge gate

Rewires loadAdvCharacter / createAdvCharacter to source from player_meta,
drops every legacy-table fallback in the loadXxxState helpers, and adds a
GOGOBEE_LEGACY_PURGE=1 gate to drop the now-cold adventure_characters
table. Schema CREATE removed and column migrations tolerate the missing
table so the purge stays sticky across reboots.

§7.4 of the migration plan is reconciled to document that
player_meta.combat_level stays — combat_stats.go consumes it via the
overlay and is shared infrastructure, not legacy.

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

internal/plugin/dnd_combat.go

@@ -282,58 +282,6 @@ func autoBuildCharacter(userID id.UserID, char *AdventureCharacter) *DnDCharacte
 	return c
 }
 
-// backfillDnDCharactersFromAdv is the L5g one-shot mass-backfill: for every
-// adventure_characters row that has no corresponding dnd_character row, build
-// an auto-migrated D&D character (same shape as ensureDnDCharacterForCombat's
-// fresh path) and persist it. After this lands, every legacy player has a
-// D&D row, so dndLevelForUser / rivalLevelForUser / hospitalCostsForUser can
-// drop their dndLevelFromCombatLevel fallbacks.
-//
-// Idempotent: skips users who already have any dnd_character row (including
-// pending-setup drafts — those finish via !setup confirm and shouldn't be
-// overwritten by the backfill).
-func backfillDnDCharactersFromAdv() error {
-	rows, err := db.Get().Query(`
-		SELECT a.user_id
-		FROM adventure_characters a
-		LEFT JOIN dnd_character d ON d.user_id = a.user_id
-		WHERE d.user_id IS NULL
-	`)
-	if err != nil {
-		return err
-	}
-	var userIDs []id.UserID
-	for rows.Next() {
-		var uid string
-		if err := rows.Scan(&uid); err != nil {
-			rows.Close()
-			return err
-		}
-		userIDs = append(userIDs, id.UserID(uid))
-	}
-	rows.Close()
-
-	created := 0
-	for _, uid := range userIDs {
-		char, err := loadAdvCharacter(uid)
-		if err != nil || char == nil {
-			continue
-		}
-		c := autoBuildCharacter(uid, char)
-		c.AutoMigrated = true
-		c.PendingSetup = false
-		if err := SaveDnDCharacter(c); err != nil {
-			slog.Error("dnd: L5g backfill save failed", "user", uid, "err", err)
-			continue
-		}
-		_ = initResources(uid, c.Class)
-		_ = ensureSpellsForCharacter(c)
-		created++
-	}
-	slog.Info("dnd: L5g backfill complete", "rows", created)
-	return nil
-}
-
 // ── Roll summary line ────────────────────────────────────────────────────────
 
 // dndRollSummaryLine scans a CombatResult's events for d20 rolls and returns