feat(discovery): drop Shorts and livestreams before the caption fetch (ADR-023)

The scarce resource is the per-IP timedtext caption fetch (ADR-014); the pilot's
candidate set was mostly Shorts/clips/livestreams, each burning a fetch (a "none"
result is a completed fetch — it costs budget even when it yields nothing).

NewVideos now enriches candidates with one cheap Data API videos.list call
(contentDetails.duration + snippet.liveBroadcastContent — the quota API, a
DIFFERENT limit from the timedtext 429) and drops, before returning: videos
shorter than TAPIR_MIN_VIDEO_SECONDS (default 60) and any live/upcoming
broadcast. Dropped videos are never persisted, so the list declutters too.

Degrade-open: MinVideoSeconds=0 disables it (no quota call); a videos.list error
returns candidates unfiltered so discovery never breaks on a metadata hiccup. The
paste-a-URL path (VideoByID) is not filtered — an explicit request is honoured.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/homelab-integration.md

@@ -39,6 +39,11 @@ it** — endpoints and aliases drift, and this file is a snapshot (2026-06-06),
   - `TAPIR_MAX_TRANSCRIPT_CHARS` — transcript truncation budget sent to the model. **Default
     `18000`** (~fits an 8k-context model). `0` disables truncation. Prevents the context-overflow
     HTTP 400 a long transcript caused on `phi4-mini`.
+- **Discovery low-value filter (ADR-023).** `TAPIR_MIN_VIDEO_SECONDS` — **default `60`**. At
+  discovery, `NewVideos` enriches candidates with one cheap `videos.list` call (quota API, NOT
+  the timedtext 429 path) and drops videos shorter than this plus any live/upcoming broadcast,
+  so the scarce caption-fetch budget isn't spent on Shorts. `0` disables the filter. The
+  paste-a-URL path is never filtered.
 - **Thinking models need an explicit `max_tokens`.** qwen3 / deepseek-r1 spend the budget on
   reasoning and return **empty content** if `max_tokens` is too low (or unset). The summarizer's
   parser treats an empty summary as an error for exactly this reason. **Done (2026-06-02, Worker F):**