Geo: Don't assume all source records are replicable in the Self-Service Framework

The following discussion from !34248 (merged) should be addressed:

We currently assume none of the replicable-related scopes apply to the primary-side, but some should. It looks like this will also impact at least artifacts after it is migrated since the not_expired scope can apply on the primary-side.

One way to resolve this is to e.g. add primary_replicables which we can reuse in:

• replicables_for_geo_node (which would return primary_replicables if node.primary? else apply primary & secondary scopes)
• primary_total_count (exposed in the primary GeoNodeStatus and prometheus metrics)
• Primary-side checksum success/failure counts and percentages (exposed in the primary GeoNodeStatus and prometheus metrics)
• ReplicableModel#needs_checksum? (I guess this should check if it's in replicables_for_geo_node since it will probably be reused during verification implementation)

(Also same for the Merge Request Diffs has_external_diffs scope)

Also, !34248 (diffs, comment 404842890):

thought: WDYT about raising if not stored externally? The callers of local? reasonably assume a falsey value means the file is remotely stored.

suggestion: I think we need to modify logic which depends on local? to not call local? if not stored externally. E.g. needs_checksum? and checksummable? should return falsey and not call local?.