chore(env): document required env vars, remove unused Pexels integration

[natashaannn]

User story

As a developer setting up a new deployment, I want a complete .env.example that documents every env var the app actually uses, and I want unused integrations removed, so that configuration is straightforward and the codebase reflects current reality.

Background

Two problems:

1. .env.example currently only documents PEXELS_API_KEY, but several env vars added by Issues feat(auth): add modular Resend subscriber email gate for carousel and get-youtube-captions #77 and feat(api): add secret token guard to internal filesystem-writing API routes #79 (RESEND_API_KEY, SUBSCRIBER_COOKIE_SECRET, etc.) need to be documented.
2. The Pexels integration (scripts/fetch-pexels.js, scripts/lib/resolve-pexels.js) is unused and was gated behind PEXELS_API_KEY being set. It should be removed along with the env var.

Acceptance criteria

Happy path

Given this issue is merged
When a developer reads .env.example
Then every env var the app uses is documented with its purpose, whether it is required or optional, and which feature area it belongs to — and PEXELS_API_KEY is absent

Given this issue is merged
When a developer runs grep -r "pexels\|PEXELS" . --include="*.ts" --include="*.js" --include="*.tsx" (excluding node_modules)
Then zero results are returned

Edge case

Given a developer has PEXELS_API_KEY in their local .env.local
When they run edit-transcript.js after this issue is merged
Then no Pexels-related code runs (it has been removed) and no error occurs

Out of scope

• Changing any non-Pexels application logic
• Adding runtime env var validation beyond what already exists
• Secret management tooling

Technical context

Files to DELETE:

• scripts/fetch-pexels.js
• scripts/lib/resolve-pexels.js

Files to EDIT:

• scripts/edit-transcript.js — remove the PEXELS_API_KEY conditional block (~lines 2030–2036) and any import of resolve-pexels.js
• package.json — remove the pexels:fetch script entry
• .env.example — remove PEXELS_API_KEY; add all new env vars grouped by feature

Implementation details

1. Delete Pexels files

rm scripts/fetch-pexels.js
rm scripts/lib/resolve-pexels.js

2. Edit scripts/edit-transcript.js

Remove the conditional block starting at ~line 2030 that checks process.env.PEXELS_API_KEY and imports resolve-pexels.js. Confirm no other Pexels references remain in the file.

3. Edit package.json

Remove the "pexels:fetch" entry from the scripts block.

4. Rewrite .env.example with all current env vars, grouped by feature:

# ── Carousel tool ─────────────────────────────────────────────────────────────
# Required for AI-powered features
ANTHROPIC_API_KEY=
# Optional: defaults to claude-sonnet-4-20250514
# CLAUDE_MODEL=claude-sonnet-4-20250514

# ── Subscriber email gate ──────────────────────────────────────────────────────
# Required: Resend API credentials for subscriber enrolment and welcome emails
RESEND_API_KEY=
RESEND_FROM_EMAIL=hello@yourdomain.com
RESEND_SEGMENT_ID=
# Comma-separated topic IDs to subscribe new contacts to
RESEND_TOPICS=
# Min 32 characters — signs the subscriber session cookie
SUBSCRIBER_COOKIE_SECRET=

# ── Internal API guard (local/internal deployment only) ───────────────────────
# Required on internal instances: Bearer token for editor and camera save routes
INTERNAL_API_SECRET=

Additional test scenarios

• grep -r "pexels\|PEXELS" scripts/ app/ remotion/ — zero results after merge
• npm run dev starts without errors after removing PEXELS_API_KEY from local .env.local
• package.json scripts block no longer contains pexels:fetch

Hard constraints

• .env.example must never contain real secret values — only placeholder strings or comments
• Variable names must exactly match what the code references (case-sensitive)
• No other scripts/ or app/ behaviour should change beyond the Pexels removal

Dependency issues

• Should be merged after Issues feat(auth): add modular Resend subscriber email gate for carousel and get-youtube-captions #77 and feat(api): add secret token guard to internal filesystem-writing API routes #79 have established their env var names
• Any env var name changes in those issues must be reflected here before this is merged