docs: fix stale FaceTheory ISR schema wording + env var naming note

[aron23]

Problem
docs/facetheory/isr-cache-schema.md still contains stale wording:

• It references docs/facetheory/isr-transaction-recipes.md “once FT-T2 lands”, but the transaction recipes doc already exists.

There’s also some avoidable friction around env var naming across repos:

• TableTheory FaceTheory docs standardize on FACETHEORY_CACHE_TABLE_NAME.
• AppTheory’s CDK construct currently wires APPTHEORY_CACHE_TABLE_NAME (+ generic aliases). (We’re opening a sibling issue to also wire FACETHEORY_CACHE_TABLE_NAME.)

Where

• docs/facetheory/isr-cache-schema.md (around the “Transactions” section)

Changes

• Update the “see transaction recipes” wording to point to docs/facetheory/isr-transaction-recipes.md directly.
• Add a short note on env var conventions:
  • FACETHEORY_CACHE_TABLE_NAME is canonical for FaceTheory ISR
  • mention AppTheory aliasing (or link to the AppTheory issue) so users don’t get stuck

Acceptance Criteria

• Docs read cleanly (no future-tense roadmap wording for already-shipped docs).
• Links are correct.

[aron23]

Implemented in PR #73 (base: staging).

[aron23]

Verified in TableTheory v1.4.0: isr-cache-schema.md now links directly to isr-transaction-recipes.md and includes an env var note for FACETHEORY_CACHE_TABLE_NAME.