feat(tickers): record delisting instead of deleting the symbol

Retiring a symbol meant delete_ticker or bootstrap_universe(prune_missing),
both of which cascade through OHLCV, setups and scores. That destroys exactly
the history four research documents already apologise for: today's tracked
universe projected backward is survivorship-biased, and hard-deleting every
delisted name is what causes it. Keeping the rows preserves the option to fix
that — it does not fix it, which needs the replay to model a delisting as an
exit event.

tickers gains delisted_on / delisted_reason (migration 032). NULL means
actively traded.

The filter is opt-in via ticker_service.active_only rather than folded into a
shared getter: the registry and admin views deliberately keep delisted rows so
the delisting is visible, and a silent default would undo that. Applied to the
live path only — scanner, momentum ranking, scoring, breadth, fundamentals
candidates, SEC universe, earnings import, ingestion loops. run_backtest keeps
them on purpose.

Detection runs off OHLCV staleness, not off the SEC fundamentals import: that
importer stalls for days on unrelated Company-Facts gaps and would take
detection down with it. On a stale symbol the scheduler asks SEC for a Form
25/25-NSE/15 and retires it only on a hit, so a halt or a rename (SATS->ECHO)
keeps the existing warning. The probe waits 3 stale days so a market-data
outage cannot turn into one SEC request per symbol per run.

Safe to automate because it is reversible: clear_delisted un-retires a false
positive, where a delete had already taken the history.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/models/ticker.py

@@ -1,6 +1,6 @@
-from datetime import datetime
+from datetime import date, datetime
 
-from sqlalchemy import String, DateTime
+from sqlalchemy import Date, String, DateTime
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.database import Base
@@ -21,6 +21,13 @@ class Ticker(Base):
     cik: Mapped[str | None] = mapped_column(String(10), nullable=True)
     sic: Mapped[str | None] = mapped_column(String(4), nullable=True)
     sic_description: Mapped[str | None] = mapped_column(String(160), nullable=True)
+    # Delisting is recorded, never deleted: the rows carry the price history that
+    # makes a backtest less survivorship-biased, and a delete cascades it away.
+    # NULL == actively traded. The live signal path filters on this (see
+    # ticker_service.active_only); list/admin views keep the row and show it.
+    delisted_on: Mapped[date | None] = mapped_column(Date, nullable=True, index=True)
+    # How we learned: "form_25" (SEC confirmed), "manual" (operator).
+    delisted_reason: Mapped[str | None] = mapped_column(String(32), nullable=True)
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), default=datetime.utcnow, nullable=False
     )