Stats: show subscriber acquisition source

To show value as a platform and help publishers optimize their site for getting more subscribers, we should better communicate customer how they gained the subscriber. Was it subscribe modal, welcome overlay, blogroll, recommendation by other publisher, Reader, etc?

Especially when we have "network effect" type features, these stats will be important for showing the network effect's impact.

#### Context, internal references:
- p58i-exh-p2
- p58i-eHZ-p2
- pedxs5-9u-p2
- peKye1-CC-p2

### Designs

Figma: S4xGQTEkLhYdUvcIdn2Cfr-fi-1_32846

i1: p1HpG7-v7v-p2
i2: p1HpG7-vch-p2

Start with the linked Figma designs and the listed internal references to understand the intended acquisition-source views. No source files, tests, or entry points are named in the issue; completion would require implementing the requested subscriber-source reporting for the examples listed, including network-effect sources.

Written by the indexing model from the issue text.