N2/B1+B4+C4: tempering, the expedition achievement wing, arena seasons

B1 — tempering. `!adventure temper` pushes an owned magic item one rung up
the rarity ladder at the blacksmith, capped at Legendary. Rarity lives on
the registry definition, so an upgraded instance records its progress as a
`temper` step count on its own row (adventure_inventory, magic_item_equipped)
and effective rarity is derived on read. The stored base rarity is never
written back, so an item cannot double-bump across a load/save round-trip.
Effects flow through the existing rarity scalars — no new combat math, and
the Legendary cap means this accelerates reaching the current ceiling rather
than raising it.

Costs mirror the crafting ladder (10k/25k/60k/150k, Foraging 15/20/25/30).
Only the Legendary rung consumes a T5 material; gating the lower rungs on
one would make tempering unreachable until a player already clears T5.

Two plan anchors were wrong: thyraks_core and portal_fragment are not
loot-note strings needing promotion — they are UniqueAlways slate entries
that already materialize as inventory rows on every T5 clear.

B4 — expedition achievement wing: first-clear and all-zones-cleared per tier
(10), a quiet-clear for keeping peak threat under 50, and temper_legendary.
The quiet-clear requires max_threat_seen to be *present*, not merely low:
recordMaxThreat samples only on day rollover and never stores a zero, so an
absent key means no threat history, not low threat. The plan's no-death-T4
achievement is not shipped — no per-expedition death counter exists — and
pet-saved-my-life already ships as combat_pet_save.

C4 — arena seasons. Standings are derived from arena_history.created_at per
calendar quarter rather than wiping arena_stats: same visible reset, but
lifetime totals survive for `!arena stats` and no destructive job can fire
twice. Crowns archive to arena_season_titles rather than player_meta.title,
which already carries the Survivalist milestone. Rollover rides the existing
midnight ticker and self-dedups on the season key, so a bot that was down on
the boundary catches up on its next wake.

internal/db/db.go

@@ -336,6 +336,15 @@ func runMigrations(d *sql.DB) error {
 		// URL link previews now post the page's og:image/twitter:image
 		// thumbnail; cache it alongside the title/description.
 		`ALTER TABLE url_cache ADD COLUMN image_url TEXT NOT NULL DEFAULT ''`,
+		// Tempering (gogobee_engagement_plan.md B1). A magic item's rarity
+		// lives on its registry definition, so an upgraded instance needs
+		// somewhere of its own to record how far it has been pushed. temper
+		// counts rarity steps above the definition's base; effective rarity
+		// is derived at the effect/render/sell boundary, never written back.
+		// DEFAULT 0 is the correct value for every pre-existing row, so this
+		// needs no bootstrap backfill.
+		`ALTER TABLE adventure_inventory ADD COLUMN temper INTEGER NOT NULL DEFAULT 0`,
+		`ALTER TABLE magic_item_equipped ADD COLUMN temper INTEGER NOT NULL DEFAULT 0`,
 	}
 	for _, stmt := range columnMigrations {
 		if _, err := d.Exec(stmt); err != nil {
@@ -1500,6 +1509,21 @@ CREATE TABLE IF NOT EXISTS arena_stats (
 	updated_at          INTEGER NOT NULL
 );
 
+-- Arena seasons (gogobee_engagement_plan.md C4). Season standings are DERIVED
+-- from arena_history.created_at, so arena_stats stays lifetime and no
+-- quarterly wipe ever runs. Only the awarded titles are archived here, one row
+-- per (season, kind) — the champion for that quarter, frozen.
+CREATE TABLE IF NOT EXISTS arena_season_titles (
+	season     TEXT NOT NULL,
+	kind       TEXT NOT NULL,          -- 'earnings' | 'streak'
+	user_id    TEXT NOT NULL,
+	value      INTEGER NOT NULL,
+	awarded_at INTEGER NOT NULL,
+	PRIMARY KEY (season, kind)
+);
+CREATE INDEX IF NOT EXISTS idx_arena_season_titles_user ON arena_season_titles(user_id);
+CREATE INDEX IF NOT EXISTS idx_arena_history_created ON arena_history(created_at);
+
 -- Rival System
 CREATE TABLE IF NOT EXISTS adventure_rival_records (
 	user_id      TEXT NOT NULL,