Fix inflated new-episodes notification count and rename to "episodes" (issue #60)

[mapitman]

Summary

• Root cause: `FeedRepository.trimToItemsToKeep` deletes evicted episodes from the DB outright, so a feed whose upstream RSS lists more episodes than `itemsToKeep` (default 20 -- common for long-running shows) re-"discovers" its own previously-trimmed episodes as new on every single refresh: each one gets re-inserted, counted as new, then immediately re-evicted again in the same pass. This inflated the notification's count indefinitely instead of settling once genuinely new episodes were seen.
• `FeedUpdateResult.Success.newItemCount` now excludes items evicted in the same cycle they were "discovered" in (`newItemIds.count { it !in evictedItemIds }`) -- both lists already come from the same `persist()` call for that feed. `newItemIds` itself is untouched, since `AutoQueueAndDownloadEnforcer` also reads it for auto-queue/auto-download, which is out of scope here.
• Renamed the notification's wording from "item(s)" to "episode(s)" throughout (title, body, channel name/description) and renamed the backing string resources (`notification_new_items_` -> `notification_new_episodes_`), across all 5 locales. The Android notification channel ID itself (`"new_items"`) is left unchanged to avoid orphaning existing users' channel settings.

Fixes #60

Test plan

• `./gradlew assembleDebug testDebugUnitTest lintDebug` all pass
• Added `FeedUpdateEngineTest.updateFeed_reDiscoveringAPreviouslyTrimmedItem_doesNotCountItAsNewAgain`, reproducing the exact trim-then-rediscover cycle and asserting the second cycle's `newItemCount` is 0
• Verified live on an Android emulator: subscribed to a real podcast feed (NPR's Planet Money, ~100+ episodes upstream), capped it to 5 items via Feed Properties, forced a refresh -- confirmed no notification fired despite ~15 historical episodes being reinserted-then-evicted in that pass (previously this would have shown "~15 new items"). Then deleted one of the 5 currently-kept episodes to simulate a genuinely new one and re-ran: notification correctly read "New episodes" / "1 new episode".