Add duplicate-project tests, data deletion docs, split lifecycle diagram (#904, #907, #903)

[annacodervibe]

Closes #904
Closes #907
Closes #903

Three issues bundled together since they're all doc/test additions on the same repo.

#904 - the existing test_create_project_fails_duplicate_id only checked the error code, not the two things the acceptance criteria actually asked for. Added two tests: one that snapshots a project's full state, attempts a duplicate with deliberately different data, and verifies the original is completely unchanged; another proving a different caller can't hijack an existing project_id.

Couldn't get a clean local cargo test run for this - traced it down to real, pre-existing breakage: 77 compile errors across several other test files (address_validation_tests.rs, reliability_tests.rs, events.rs, ttl_renewal_tests.rs, cost_benchmark_tests.rs), none of which I touched. Stale API names (add_allowed_token should be allow_token), missing lifetime annotations, methods that don't exist anymore. None of it's in or near my two new tests - the compiler doesn't even get that far. Confirmed this on a completely clean checkout of main too, unrelated to this PR. Happy to have CI or a reviewer confirm the two new tests pass once that's sorted separately.

#907 - wrote up the data deletion/account closure process. Worth knowing: transactions has no FK to users at all, it's keyed on wallet address, which matters for what "delete a user" can even mean here. Doc covers wallet-signature-based identity verification (matches the existing no-password auth model), and a clear table of what gets deleted vs anonymized vs retained and why.

#903 - added a lifecycle sequence diagram (create/fund/distribute/observe) as a companion to the existing static architecture diagram. Grounded in the actual contract functions and the real backend event-polling code, not guessed.

On the CI checks: Markdown Format Check and Markdown Link Check run against the entire docs/ and runbooks/ tree, not just this PR's files. Confirmed on a fresh clone of main with zero changes that 93 files already fail the format check before this PR touches anything - so these two checks can't go green without fixing all of that unrelated, pre-existing content, which is out of scope here. I did fix formatting and a broken anchor link in my own 2 new files specifically.

The other failing checks (Contract Artifact Size, Docker Compose Health Check, Frontend CI/Quality, Migration Dry-Run, CodeQL, data-integrity) don't have any plausible path through a 2-file doc change and 2 new Rust test functions - haven't traced each one individually, but given the pattern above, these look like other pre-existing, repo-wide issues rather than anything this PR introduced.

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