fix(server): log an unreadable config at warn, not error

[hyperpolymath]

What

_read_databases (src/server/routes/databases.jl) and _read_primers
(src/server/routes/config.jl) both log at @error with a full backtrace, then
answer json_error(400, ...). This changes those two to @warn and keeps the
reason string.

Why this is a fix and not a mute

The status the route already assigns is the discriminator. json_error(400, ...)
is the function's own verdict that a malformed config/databases.yml or
config/primers.yml is an operator-fixable input, not a server fault.
Logging that at error severity with a stack trace reports a routine bad file as
if the server had crashed.

The backtrace also carried no diagnostic value here: it pointed into YAML.jl's
parser, never at the offending line. What actually names the defect is
sprint(showerror, e), and that is kept as reason=. Only the backtrace goes.

Deliberately NOT changed — 3 of the 5 route @error sites stay as they are

site	status	disposition
routes/databases.jl:31	400	→ @warn
routes/config.jl:272	400	→ @warn
routes/analysis_config.jl:144	500	unchanged
routes/annotations.jl:318	500	unchanged
routes/composition.jl:370	500	unchanged

A 500 is the route saying it did not expect this. There the backtrace is the
only thing separating a data problem from a code bug, so dropping it would be
muting a diagnostic rather than repairing one.

analysis_config.jl already draws exactly this line itself: its ArgumentError
branch returns 400 with no logging at all, while everything else logs at
@error and returns 500. That is the existing precedent this PR follows rather
than overrides.

src/server/routes/pipeline.jl:520 is untouched — it sets merged_results[i] = nothing
inside a Threads.@threads loop with no json_error at all. That is the
silent-degradation shape tracked in #31, not a severity question.

Verification

• Both files checked with Meta.lower after editing, not just Meta.parseall —
  a lowering error (e.g. an orphaned break) passes parseall silently.
• No test asserts on these sites. The suite's only two @test_logs are
  (:warn, ...) on filter_table and _apply_single_mapping! in
  TaxonomyTableTools; neither reaches _read_databases or _read_primers, and
  nothing matches databases_unreadable / primers_unreadable / is not readable.
• Cut from main; touches only src/, so it could not conflict with fix: clear all 27 SonarCloud findings and unblock the Julia analysis-config step #29
  (frontend-only apart from one test file).

🤖 Generated with Claude Code

https://claude.ai/code/session_01X3hgXxWm6umMgZkjYyHnnm

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📥 Commits

Reviewing files that changed from the base of the PR and between f699295 and 33aad5a.

📒 Files selected for processing (2)

• src/server/routes/config.jl
• src/server/routes/databases.jl

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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