feat(x): viral live-prediction X auto-poster for Trump signals

When Trump posts an actionable buy/short signal, instantly tweet the call
(entry/direction, hard time window, "ratio me" hook), then post a result
follow-up X_FOLLOWUP_MINUTES later reporting the realised 15-min move plus
a recent hit-rate brag with the site link.

- New app/services/x_poster.py: OAuth 1.0a signed (stdlib hmac, no new dep,
  httpx reused). Fire-and-forget, fully degradable — no-op unless X creds +
  x_enabled. Dry-run logs tweet text. In-memory UTC-daily cap. 280-char hard
  cap on all templates (no Premium).
- Gated to source=truth buy/short above x_min_confidence; scanner signals
  excluded (don't fit the "Trump just posted" format).
- Hooked next to notify_signal in /signals/ingest + both Trump pollers
  (dedup via unique external_id → single tweet per post).
- config.py + .env.example: X_API_KEY/SECRET, X_ACCESS_TOKEN/SECRET,
  X_ENABLED, X_MIN_CONFIDENCE, X_DAILY_CAP, X_FOLLOWUP_MINUTES, X_LINK_URL.

72 tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/api/signals.py

@@ -219,6 +219,14 @@ async def ingest_signal(
     except Exception as exc:
         logger.warning("Telegram notify failed for signal %d: %s", post.id, exc)
 
+    # Auto-post a viral "live prediction" tweet (fire-and-forget; no-op unless
+    # X is configured + enabled). Internally gated to source=truth buy/short.
+    try:
+        from app.services.x_poster import notify_x_signal
+        notify_x_signal(post)
+    except Exception as exc:
+        logger.warning("X notify failed for signal %d: %s", post.id, exc)
+
     return SignalIngestResponse(status="accepted", post_id=post.id)