fix(config): refuse a negative pool_size in redis.url

[blurbery]

Problem

Closes #1862
Related issue: #1861
Validation tasks: none

A redis.url with a negative pool_size (for example redis://redis:6379?pool_size=-1) passes validation, and go-redis panics when a client is built from it. The admin connection check answers 500 with a handler panic, the save is accepted, and the next start panics before anyone can fix the value in the admin UI. #1862 has the full reproduction. This change refuses a negative pool_size when the URL is parsed, so the save answers 422 naming the option and the connection check reports a failed check, as they already do for pool_size=many.

go-redis replaces a pool_size of 0 with its default, but uses any other value as the size of its pool's semaphore channel, so a negative one panics in NewClient with makechan: size out of range. Since Sentinel URLs became supported (#1859), the same happens for them: the master client gets the same pool size.

Approach

• ParseRedisURL returns redis: pool_size must be 0 or more; leave it out for the default for a negative pool_size, for both single-server and Sentinel URLs. NormalizeRedisURL, the connection check and startup all go through it, so the save refuses it and nothing builds a client from it.
• Other negative pool options (min_idle_conns, max_idle_conns, max_active_conns, max_concurrent_dials) are left alone. go-redis ignores or clamps them, and refusing them would stop a server that starts today, because startup exits when its saved URL can't be used (A saved redis.url that Silo cannot connect with stops the server at the next start #1861).

A server that already has pool_size=-1 saved still can't start, but with a clear error instead of a panic. Recovering from a saved URL that doesn't work is #1861.

Validation

• Tests in internal/config refuse pool_size=-1 in single-server and Sentinel URLs, check the error names the option, build a go-redis client from every pool size ParseRedisURL accepts (0 and 20 still work), and check NormalizeRedisURL refuses it. On ca186fe they fail: both URLs parse, building a client panics with makechan: size out of range, and the save accepts it.
• go test ./internal/config/ passes. go vet and make lint-changed: clean.
• CI on 9165365: every job passed (run).
• Not run on a server with this change.

Benchmarks

Not applicable. This only adds a check when the URL is parsed.

Evidence

Evidence: https://evidence.siloserver.org/r/silo-server/pr-2079/

Before: the reproduction in #1862 and the failing tests. After: the save refuses the URL with an error naming pool_size.

Risks

None identified. A negative pool_size could never build a client, so no working setup is refused.

Checklist

• I read and can explain the complete diff.
• This pull request addresses one concern.
• The Evidence section shows every change a user can see, or says there is none.

AI Disclosure

• Harness: Claude Code (desktop app)
• Tool(s): Claude Code, go, gh
• Model(s): claude-opus-5-5
• Involvement: AI-assisted
• Adversarial review: n/a, small validation change

AI-assisted with Claude Opus. I directed the task and designed the work.

[silo-kody]

Silo Kody — review complete

Review finished. Check the inline comments for findings and verify each suggestion against the code and tests.

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📥 Commits

Reviewing files that changed from the base of the PR and between ca186fe and 9165365.

📒 Files selected for processing (2)

• internal/config/admin_settings.go
• internal/config/redis_url_test.go

• Autopilot · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts

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[macroscopeapp]

• Visible effect: Saving a Redis URL with negative pool_size now produces a specific validation error in the web admin (internal/config/admin_settings.go); the Evidence section has no published link or captures. Evidence needed: before-and-after captures of the same Redis settings save with that input on desktop and phone-width web, identifying the surface and each build/commit.
• Suggested fix: Attach those captures to the PR or publish them under Evidence with a valid evidence.siloserver.org link.

_{Automated check: Macroscope check run agent (gpt-6-luna). Evidence was not reviewed for correctness.}

^{Posted via Macroscope — Visible change evidence}