This commit improves handling of missing channel signatures in the
database:
- Return nil from auth proof accessors instead of empty slices so that
missing signatures are stored as NULL in SQL.
- Update public channel checks to require signature length > 0, which
properly handles existing empty bytea values in the database.
- Add regression test covering empty v1 and v2 channel signatures to
prevent future issues.
Also update it to more closely match the persisted version which has the
v1 and v2 only fields as optional.
Refactor ChannelAuthProof to support both v1 and v2 channel
announcements:
- Add Version field to distinguish v1 from v2 proofs
- Wrap v1-specific fields (NodeSig1/2, BitcoinSig1/2) in fn.Option
since v2 doesn't use them
- Add optional Signature field for v2's single schnorr signature
- Add constructor functions NewV1ChannelAuthProof and
NewV2ChannelAuthProof to enforce correct initialization
- Add getter methods (NodeSig1(), BitcoinSig1(), etc.) that safely
unwrap options, returning empty slices when not present
The IsEmpty() check is updated to handle both versions correctly.
Both stores validate v1-only for now.
All the structs defined in the `channeldb/models` package are graph
related. So once we move all the graph CRUD code to the graph package,
it makes sense to have the schema structs there too. So this just moves
the `models` package over to `graph/db/models`.
Elle Mouton