Fix game timer: cycle-key tracking, wall-clock only, active gate

games_sp.log is not wiped on restart, so old J; lines from prior
sessions were poisoning the anchor. Now the scanner walks the tail
backward to the LATEST InitGame and only considers lines after it,
returning an opaque cycle key (the first J; line) plus a state. The
worker stamps game_started_at/ended_at with Date.now() at observation
time instead of inferring from the log's own timestamps. Same-players
game restarts produce a new key and cleanly re-anchor. When the server
transitions to 0 players, game state is cleared server-side and the
UI also gates the timer display on activeSince.

Adds servers.game_cycle_key column with additive migration.

src/lib/server/db/schema.sql

@@ -38,8 +38,9 @@ CREATE TABLE IF NOT EXISTS servers (
     -- and resume from the same anchor when the server comes back.
     active_since     TEXT,                                -- ISO ts of last 0 -> >0 player transition
     inactive_since   TEXT,                                -- ISO ts of last >0 -> 0 player transition
-    game_started_at  TEXT,                                -- ISO ts we first observed InitGame: for the current match
-    game_ended_at    TEXT,                                -- ISO ts we observed ExitLevel/ShutdownGame ending it
+    game_started_at  TEXT,                                -- wall-clock ISO when we first saw a J; line for the current match cycle
+    game_ended_at    TEXT,                                -- wall-clock ISO when we first saw ExitLevel/ShutdownGame end it
+    game_cycle_key   TEXT,                                -- opaque marker for the currently-tracked cycle (the first J; line after the latest InitGame); lets us detect same-players game restarts
     created_at       TEXT NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%fZ','now'))
 );