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/config.py

@@ -64,6 +64,32 @@ class Settings(BaseSettings):
     # (no trading details, no /adopt CTA). Leave empty to disable.
     telegram_public_channel_id: str = ""
 
+    # ── X (Twitter) auto-poster ──────────────────────────────────────────────
+    # OAuth 1.0a User Context — for posting from a single fixed bot account
+    # (@TrumpAlpha_signals). Get all four from developer.x.com → your App →
+    # "Keys and tokens". App permissions MUST be "Read and Write", and the
+    # Access Token/Secret must be regenerated AFTER setting write permission.
+    # Bearer Token / Client ID / Client Secret are NOT used (those are read /
+    # OAuth2 web-flow credentials). Empty = X posting disabled (no-op).
+    x_api_key: str = ""              # = Consumer Key
+    x_api_secret: str = ""           # = Consumer Secret
+    x_access_token: str = ""
+    x_access_secret: str = ""
+    # Master switch. Even with keys set, posting stays off unless this is true.
+    # Lets you load creds without going live (dry-run = logs the tweet text).
+    x_enabled: bool = False
+    # Only post when ai_confidence >= this (0–100). Keeps low-quality noise off
+    # the public timeline. Scanner signals carry their own score.
+    x_min_confidence: int = 60
+    # Hard daily cap on tweets (initial + follow-ups counted together) to stay
+    # under Free-tier 1,500/mo and avoid spam flags. In-memory, resets at UTC
+    # midnight. Single-process by design so a plain counter is safe.
+    x_daily_cap: int = 40
+    # Minutes after the initial prediction tweet to post the result follow-up.
+    x_followup_minutes: int = 15
+    # Public site URL appended to the follow-up tweet. Falls back to frontend_url.
+    x_link_url: str = ""
+
     model_config = {"env_file": ".env", "env_file_encoding": "utf-8", "extra": "ignore"}