feat: Add status bar with model and usage information

[BinaryMuse]

This PR adds a status bar that includes the currently selected model on the left and the user's overall AI usage on the right, if it's over a certain threshold, as well as a short string (either in whole number days, hours, or minutes) describing when the limit will reset.

The threshold is currently set to 50%, and it changes color as usage gets higher.

The percentage is based on either input or output usage, whichever is higher; e.g., if a user has 40% input usage and 80% output usage, the widget will show 80%.

Usage is cached for 60 seconds in the AI database, and if stale, is updated in the background when the TUI launches. Each turn ends in an event that sends updated usage information, so the widget will remain correct over the course of a session.

[greptile-apps]

Greptile Summary

This PR adds a status bar to the AI TUI showing the currently selected model and credit usage when it exceeds 50%, with color-coded bar and reset countdown. Usage snapshots are fetched on TUI open (with 60s cache), refreshed in the background if stale, and updated at the end of each turn via the done event.

• New usage.rs module handles snapshot deserialization, cache-key generation, background HTTP fetch, and the percentage/reset-delta helpers.
• A new usage SQLite table caches the snapshot per hashed auth token; AiSessionStore and SessionService gain get_usage/set_usage methods.
• StreamControl::Done is extended to carry an optional UsageSnapshot from the server, and DriverEvent::Usage propagates it to the view state without touching the FSM.

Confidence Score: 5/5

Safe to merge — changes are additive and well-isolated from existing functionality.

The usage snapshot path is entirely additive: a missing or stale cache is a no-op for rendering, fetch failures are swallowed with a debug log, and the done-event credits field is parsed leniently. No existing code paths are altered in a breaking way.

No files require special attention.

Important Files Changed

Filename	Overview
crates/atuin-ai/src/usage.rs	New module: UsageSnapshot type, percentage/reset helpers, background fetch, cache key. Tests pass and logic matches PR description (max of input/output, not average).
crates/atuin-ai/src/driver.rs	Adds DriverEvent::Usage variant and update_usage helper; uses same block_on pattern as existing persist() function. ViewState gains optional usage field.
crates/atuin-ai/src/tui/view/mod.rs	Adds status_bar_view with model label and color-coded usage bar; width calculation correctly accounts for all spans including Unicode bar characters.
crates/atuin-ai/src/store.rs	Adds get_usage/set_usage (upsert) to AiSessionStore with correct parameterized SQL and a roundtrip test.
crates/atuin-ai/src/commands/inline.rs	Reads cached usage at startup, sets it on initial ViewState, and spawns a background refresh task when the cache is stale.
crates/atuin-ai/src/stream.rs	StreamControl::Done extended with optional credits field; parsed leniently with .ok() so missing/malformed credits don't break the stream.
crates/atuin-ai/src/session.rs	SessionService trait gains get/set_cached_usage methods; LocalSessionService delegates to AiSessionStore. CachedUsageSnapshot wrapper added.
crates/atuin-ai/migrations/20260707000000_create_ai_usage.sql	Creates usage table with user_key PK; uses IF NOT EXISTS for safety.
crates/atuin-ai/src/lib.rs	Registers new usage module.

_{Reviews (2): Last reviewed commit: "Return CachedUsageSnapshot from get_cach..." | Re-trigger Greptile}

[taylordotfish]

Looks good overall, minor comment about another tuple return but I'll leave it to you to decide if it's worth changing.