Document missing setup prerequisites and env vars in README

[JSv4]

Description of the change

Investigated "issue #2104," whose reported hypothesis was a .env-configuration or setup-documentation problem. That issue number doesn't exist in this repo (Warp-Ingest is 5 days old with only 6 PRs and zero standalone GitHub issues, and searching "2104" across issues/PRs returns nothing), so I audited the actual setup docs and environment-variable handling to check whether the underlying hypothesis held up.

Findings:

• The .env theory doesn't hold as stated — there is no .env-loading mechanism anywhere in the codebase (no python-dotenv, no load_dotenv call). A .env file, correctly configured or not, would silently do nothing.
• WARP_API_KEY's abc123 fallback was already documented in the README and additionally guarded by a runtime startup warning — that part was already well handled.
• The "didn't properly document setup" half of the hypothesis does hold, narrower than expected: no stated Python version requirement, no mention that uv itself needs to be installed first, no note that the ocr/all extras need libgomp1/libmagic1 system packages outside Docker, and 7 real, working env vars (LOG_LEVEL, WARP_DISABLE_OCR, WARP_OCR_DPI, WARP_OCR_MAX_SIDE_LEN, WARP_OCR_DET_LIMIT, WARP_FE_PARALLEL_MIN_PAGES, WARP_LINE_CACHE) missing from the README's "every knob is documented here" env-var table.

This PR fixes those gaps in README.md:

• States the >=3.10,<3.15 Python requirement and adds a uv install pointer before the dev-setup commands.
• Documents the libgomp1/libmagic1 system-package requirement for a bare-metal OCR install.
• Adds the 7 missing env vars to the reference table.
• Adds an explicit one-line warning that abc123 is an insecure public default that must be overridden before any non-local deployment.

Type of change

• Bug fix (non-breaking change that fixes an issue)

Related issues

Investigated "issue #2104" — confirmed it does not exist in this repository's issue/PR tracker (0 real issues total; highest number is PR #6). This PR instead resolves the concrete documentation gaps the investigation surfaced. If #2104 actually lives in a different repository (e.g. the older Open-Source-Legal/OpenContracts), please point me there and I'll follow up.

Checklists

Development

• Lint rules pass locally (README-only change, no code touched)
• The code changed/added as part of this pull request has been covered with tests (n/a — documentation only)
• All tests related to the changed code pass in development (n/a — documentation only)

Code review

• This pull request has a descriptive title and information useful to a reviewer
• "Ready for review" label attached to the PR and reviewers mentioned in a comment
• Changes have been reviewed by at least one other engineer
• Issue from task tracker has a link to this pull request

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