refactor: change existence-check helpers to return (bool, error)

Follow-up to #4484, which kept the bool-only signature for FeedExists,
CategoryFeedExists (internal/storage/feed.go), and CategoryIDExists
(internal/storage/category.go) and instead logged a real query/connection
error via slog before returning false -- deliberately narrow, since
changing the signature would touch every call site.

This does that follow-up: all three now return (bool, error), and every
call site (14 total, across internal/api, internal/ui,
internal/reader/handler, and internal/validator) is updated to distinguish
a genuine backend failure from "resource not found":

- internal/api/*_handlers.go: response.JSONServerError on a real error,
  existing response.JSONNotFound/JSONBadRequest behavior unchanged
  otherwise.
- internal/ui/*.go: response.HTMLServerError on a real error, existing
  response.HTMLNotFound behavior unchanged otherwise.
- internal/reader/handler/handler.go: locale.NewLocalizedErrorWrapper(err,
  "error.database_error", err) on a real error, matching the exact idiom
  already used elsewhere in this same file for other storage errors.
- internal/validator/feed.go: *locale.LocalizedError (unlike
  *locale.LocalizedErrorWrapper used elsewhere) has no way to carry an
  underlying error, so a signature change here would be a separate, larger
  change to the validator package's error model. Logs the real error via
  slog and falls back to the existing "error.feed_category_not_found"
  message, same as before -- this is a deliberate scoping decision, not an
  oversight, and is called out in the PR description.

Testing: go build ./..., go vet ./..., gofmt -l (clean on all 10 touched
files), and the full go test ./... suite (48 packages, all pass, no
existing test called these 3 functions directly). No new test added, same
reasoning as #4484: no DB-error-injection harness exists for this
package, and this change doesn't alter behavior when there's no error --
every touched call site's non-error branch is behaviorally identical to
before.