Adds an optional per-chat spend limit that governs the total cost of a
chat tree (root chat plus all subagent descendants). When the cumulative
total_cost_micros across the tree reaches the limit, further messages
are rejected with a 409 containing scope: "chat_lifetime".

Database: migration 000463 adds nullable spend_limit_micros column
with a CHECK (> 0) constraint. GetChatTreeSpendTotal sums priced
messages across the root and all descendants.

Enforcement: checkChatTreeSpendLimit resolves the root chat,
fetches its limit, and compares against tree spend. Runs before the
existing account-period check in SendMessage, EditMessage, and
PromoteQueued. Fails open on lookup errors to avoid blocking users
during transient DB issues.

API: CreateChatRequest accepts optional spend_limit_micros
(validated > 0). ChatUsageLimitExceededResponse gains a scope field
(account_period or chat_lifetime) and resets_at is now optional
(nil for lifetime limits).

This PR was authored by Mux on behalf of Mike.