refactor: one definition of a followed channel and of a choice spelling

The explore purge rebuilt followed_channel's two EXISTS clauses inline, so what is polled and what may be deleted could drift; it now uses the predicate, which moved to sync/explore.py. The env validator and sysconfig each folded a choice value on their own; both call config.canonical_choice. The deep-backfill comment no longer calls recent backfill unconditional.

backend/app/sysconfig.py

@@ -14,7 +14,7 @@ from dataclasses import dataclass
 from sqlalchemy.orm import Session
 
 from app import security
-from app.config import Settings, choices_of, settings
+from app.config import Settings, canonical_choice, choices_of, settings
 from app.models import SystemConfig
 
 log = logging.getLogger("siftlode.config")
@@ -210,11 +210,10 @@ def _default(s: ConfigSpec):
 
 def _choice(s: ConfigSpec, raw: object) -> str:
     """The canonical spelling of `raw` among the key's choices (any letter case), or ValueError."""
-    folded = str(raw).strip().casefold()
-    for option in s.choices:
-        if option.casefold() == folded:
-            return option
-    raise ValueError(f"{s.key} must be one of: {', '.join(s.choices)}")
+    option = canonical_choice(s.choices, raw)
+    if option is None:
+        raise ValueError(f"{s.key} must be one of: {', '.join(s.choices)}")
+    return option
 
 
 def _coerce(s: ConfigSpec, raw: str):