Fix PUT read-state monotonicity guard

[TheGreatAxios]

Fixes CL-7131 — https://linear.app/abklabs/issue/CL-7131

Problem

putReadState (packages/chat/src/store.ts:303-323) unconditionally
set lastSeenCreatedAt/lastSeenId to whatever
PUT /workbenches/:id/read-state (packages/chat/src/routes.ts:3498-3540)
carried, via an unconditional onConflictDoUpdate. A slower "read up
to X" request landing after a faster "read up to Y" request regressed
the stored cursor, flipping messages X..Y back to unread for that
reader. The in-memory store (packages/chat/src/store.ts:461) had the
same bug.

Change

• Drizzle store: putReadState's onConflictDoUpdate now sets both
  columns via a CASE WHEN excluded.last_seen_created_at >= workbench_read_state.last_seen_created_at THEN excluded... ELSE workbench_read_state... END, so a strictly older write is a no-op
  that returns the still-current row, while an equal-timestamp write
  still lands.
• In-memory store: same rule, expressed as a plain comparison against
  the existing row before overwriting.
• The >= (not strict >) matters because workbench_messages.created_at
  has millisecond precision and same-millisecond messages are expected
  — a legitimate forward move to a later same-millisecond message must
  not be dropped as a no-op.
• The route was left unchanged — it already just echoes back whatever
  row putReadState returns, so a no-op write now correctly reports
  the still-current cursor with a normal 200.

Tests

• packages/chat/test/store.test.ts: in-memory cases for (a) a stale
  write landing after a newer one is a no-op, and (b) a same-millisecond
  forward move to a different message still lands. Both verified red
  against the old code, green after the fix.
• packages/chat/test/read-state.drizzle.test.ts: new DB-gated suite
  (skipped without DATABASE_URL, mirrors the existing
  *.drizzle.test.ts pattern) proving the same three cases against the
  real Postgres upsert. Ran locally against a local Postgres with both
  red and green results for each case.

[TheGreatAxios]

critique · comment

Read-state GET and PATCH report through reportError and return { error, refId }.

• packages/chat/src/read-state-routes.ts:89-97 and :146-157 — the 500 JSON includes refId. Tests cover the GET and PATCH reportError calls.

Walking-skeleton red on this PR is main deleting DATABASE_URL after memory-mount tests (CL-7182 / #465), not this diff.