feat(operations): cargo cache force purge ("empty all")

purge_cargo_cache gains force: bool. The default age-based purge only
ever removed entries past the configured retention window, which read
as a lie when a user expected "purge" to clear everything -- force=True
sets the cutoff a day in the future so every existing entry is treated
as expired, backing the web UI's explicit "Empty all" action. A
disabled cache stays untouched either way.

src/ci_orchestrator/operations/cache.py

@@ -94,19 +94,33 @@ def cargo_cache_status(config: Config) -> CacheStatus:
 
 
 def purge_cargo_cache(
-    config: Config, *, retention_days: int | None = None, dry_run: bool = False
+    config: Config,
+    *,
+    retention_days: int | None = None,
+    force: bool = False,
+    dry_run: bool = False,
 ) -> PurgeResult:
     """Remove cache entries older than ``retention_days`` (or config's).
 
     Sizes the victims before deleting (a real prune makes their bytes
     unreadable), so ``freed_bytes`` is accurate even for a real purge.
     A disabled cache is left untouched and reports an all-zero result.
+
+    ``force=True`` ignores the retention window entirely and empties the whole
+    cache (every entry, regardless of age) -- this backs the web UI's explicit
+    "Empty all" action, as opposed to the default age-based purge. A disabled
+    cache is still left untouched even under ``force``.
     """
     if not config.cargo_cache.enabled:
         return PurgeResult(removed_entries=0, freed_bytes=0, dry_run=dry_run)
 
-    days = retention_days if retention_days is not None else config.cargo_cache.retention_days
-    cutoff = datetime.now(tz=UTC) - timedelta(days=days)
+    if force:
+        # A cutoff in the future makes every existing entry (mtime <= now)
+        # older than it, so the age-based prune removes the entire cache.
+        cutoff = datetime.now(tz=UTC) + timedelta(days=1)
+    else:
+        days = retention_days if retention_days is not None else config.cargo_cache.retention_days
+        cutoff = datetime.now(tz=UTC) - timedelta(days=days)
     store = CargoCacheStore(config.cargo_cache.root)
 
     doomed = store.prune_older_than(cutoff, what_if=True)