[codex] fix(handlers): detect not-found with errors.Is

[prettyboyvic]

Summary

• replace fragile ecosystem lookup string comparisons with a shared errors.Is(err, pgx.ErrNoRows) mapper
• apply wrapped-error-safe detection to both ecosystem GetByID handlers and the remaining public project lookup
• cover unwrapped, wrapped, generic, and nil lookup failures with focused unit tests

Why

Matching database error text can misclassify a real not-found response as a 500 when pgx errors are wrapped or their wording changes. Stable sentinel matching preserves the API contract across wrapped error chains.

Security notes

• only pgx.ErrNoRows maps to HTTP 404
• all other failures map to a stable HTTP 500 code
• raw database error text is never returned to clients

Validation

• go test ./internal/handlers/... — passed
• audited internal/handlers/** — no string or direct-equality pgx.ErrNoRows checks remain
• git diff --check — passed

Closes #31