v0.5.0 — Plans and billing

Carl costs real money to run, so this release makes what he spends visible and bounded.

Three plans — Free, Plus, and Premium — each with its own topic count, brew frequency, and monthly model budget. Every user gets their own virtual key with that budget attached, so a brew and a chat draw on the same pot and neither can overrun it.

Spend is shown where you'd look for it: Carl's coffee fund on your account page for the month against your plan's limit, and a per-topic cost column on your profile so you can see which topic is the expensive one.