fix: remove context reaper to prevent background tabs from going stale

Background windows stopped updating because the reaper suspended contexts
after ContextSuspendAfter and fully reaped them after ContextTTL. Suspended
contexts had to re-run the page init function from scratch on reconnect,
losing the live-updating experience.

Contexts now live until the browser tab closes (beforeunload beacon) or
the server shuts down. The context map grows indefinitely — no background
reaper.

Removes: startReaper, reapOrphanedContexts, suspend/resume logic,
ContextSuspendAfter/ContextTTL config fields, lastSeenAt/suspended
context fields, and all associated tests.

docs/getting-started.md

@@ -66,7 +66,6 @@ v.Config(via.Options{
     Plugins:         []via.Plugin{MyPlugin},
     SessionManager:  sm,
     PubSub:          customBackend,
-    ContextTTL:      60 * time.Second,
     ActionRateLimit: via.RateLimitConfig{Rate: 20, Burst: 40},
 })
 ```
@@ -83,7 +82,6 @@ v.Config(via.Options{
 | `DatastarContent` | (embedded) | Custom Datastar JS bytes |
 | `DatastarPath` | `"/_datastar.js"` | URL path for the Datastar script |
 | `PubSub` | embedded NATS | Custom PubSub backend. Replaces the default NATS. See [PubSub and Sessions](pubsub-and-sessions.md) |
-| `ContextTTL` | `30s` | Max time a context survives without an SSE connection before cleanup. Negative value disables the reaper |
 | `ActionRateLimit` | `10 req/s, burst 20` | Default token-bucket rate limiter for action endpoints. Rate of `-1` disables limiting |
 
 ## Static Files