fix(credits): make credit deduction an atomic conditional decrement (#71)

[Rudra-clrscr]

Closes #71

Problem

deductCredit() in lib/database.ts performed a read-then-write:

1. read the current value via getCredits(userId)
2. write back remaining - 1 in a separate query

Two concurrent requests for the same user could both read the same value
(e.g. 1) before either wrote, and both compute and write 0 — funding two
operations from a single credit.

Fix

Move the decrement into a Postgres function so it happens in one atomic,
conditional statement.

• supabase/migration-atomic-deduct-credit.sql — adds
  deduct_credit(p_user_id UUID, p_feature TEXT), which does a conditional
  UPDATE ... WHERE <col> > 0 for the feature's _remaining column and returns
  whether a row was affected.
• lib/database.ts — deductCredit() now calls the function via
  admin.rpc("deduct_credit", ...) and keeps its Promise<boolean> return
  shape, so consumeCredit() and every other caller is unchanged.

Under row-level locking, when two calls race on the last credit the first
commits the decrement to 0 and the second re-checks the WHERE against the
updated row, matches nothing (ROW_COUNT = 0), and returns FALSE. That maps
to consumeCredit() returning { allowed: false, reason: "no_credits" }.

Acceptance criteria

• deductCredit performs an atomic, conditional decrement
• Concurrent calls with 1 credit result in exactly one success and one rejection
• Existing callers keep a compatible return shape (still boolean)

Deployment note

The migration must be applied before/with deploy. Supabase automatically
reloads the PostgREST schema so rpc("deduct_credit") is exposed. Until the
function exists the RPC errors and deductCredit returns false — i.e. it
safely denies rather than over-grants.

Testing

• tsc --noEmit: clean around the change.
• Not run live: the race behavior can only be exercised against a Supabase DB
  with the function applied. Suggested manual check — set a user's
  ai_chat_remaining to 1, fire two concurrent consumeCredit() calls, and
  confirm exactly one succeeds and the row ends at 0.

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