fix(tts): wire dashboard tts.timeout_ms through ApplySystemConfigs + seed

[vanducng]

Bug

`ApplySystemConfigs` and `seedConfigForContext` both handle `tts.provider`, `tts.auto`, `tts.mode`, `tts.max_length` — but neither carries `tts.timeout_ms`. Result: the dashboard /tts page Timeout (ms) input is inert; `cfg.Tts.TimeoutMs` stays at config.json default; `setupTTS` passes 0 to provider constructors; each provider falls back to its hardcoded default (Edge = 30s).

Real-world impact

Tenant configures Timeout=300000 (5 min) on the dashboard for long-form TTS. Saves. Trace still shows `tts failed: edge-tts failed: signal: killed` at exactly 30s. The dashboard value never reached the provider.

Fix

Two-line addition mirroring the existing pattern for the other `tts.*` keys:

• `internal/config/config_system.go`: `integer("tts.timeout_ms", &c.Tts.TimeoutMs)`
• `cmd/gateway_system_config_sync.go`: `setInt("tts.timeout_ms", cfg.Tts.TimeoutMs)`

Hot-reload (`tts-config-reload` subscriber in lifecycle) re-builds the manager + Edge provider, so the dashboard change now propagates without restart.

Test plan

• `go build ./...` + `-tags sqliteonly` + `go vet` green
• Manual: dashboard set Timeout (ms) = 300000 → save → trigger long-form TTS → edge-tts succeeds within the 300s window

[mrgoonie]

Backlog review: merge-candidate. This is a two-line config wiring fix for tts.timeout_ms through system_configs seed/apply, with go and web checks passing. It pairs well with #1175; either merge together or after #1175, but this PR itself is small and low-risk.

[mrgoonie]

review-pr --fix --reply result

Verdict: Approve.

Summary: This PR wires tts.timeout_ms through both ApplySystemConfigs and system config seeding, matching the existing tts.provider, tts.auto, tts.mode, and tts.max_length pattern.

Iterations: 1 review/verify loop, 0 fix commits.
Findings: No critical or important findings.
Merge/conflict state: GitHub reports CLEAN.
CI state: Existing PR checks are green (go, web).

Local verification run:

• go test ./internal/config ./cmd
• go build ./...
• go vet ./...
• go build -tags sqliteonly ./...

Unresolved blockers/questions: none.