[bmc08gt]

Syncs flipcash2 protos from 9f67566f to 1897108a (flipcash2-protobuf-api #140–#146) and adds the 0.19.0 changelog entry, which also covers #139 from the previous sync.

Source break: ChatMetadata.DmPayment.Action moves to a top-level PaymentAction enum. Values and field number are unchanged, so it's wire-compatible, but code naming the nested type stops compiling in both languages.

New:

• poll.v1: a Poll service with CreatePoll, GetPoll, GetPolls and Vote, plus WidgetContent.poll (a PollWidget carrying the poll ID).
• ChatMetadata.group_payment for paying a group member, and CashContent.receiver (field 3, previously reserved), which is unset on older DM messages.
• TextContent.link_previews with a new LinkPreview message.
• SystemContent.type, a required oneof starting with user_joined. fallback_text is still always set for clients that don't know the variant.
• EncryptedContent.ciphertext max grows from 17408 to 28672 bytes.

Nothing was renumbered; every result enum in this release is new.

Release this as 0.19.0 after merge, alongside ocp-client-protocol 0.7.0 (code-payments/ocp-client-protocol#10). Both app PRs consume the two packages on one branch:

• Android: feat(flipcash): adopt ocp 0.7.0 and flipcash2 0.19.0 contracts code-android-app#1781 (draft)
• iOS: feat(poll): adopt ocp 0.7.0 and flipcash2 0.19.0 contracts code-ios-app#1065 (draft)