spec: add SPEC-008 valuable error messages

Cross-cutting error-quality initiative: one structured AppError envelope
(stable error_code + message + correlation_id) replacing the current ad-hoc
mix (bare (StatusCode,&str) tuples, per-file ErrorResponse, two JSON envelopes
the dashboard already unions); correlation-id middleware tied to tracing spans
+ response header so a reported id greps the log; contextual error logging with
identifiers + error chain; sweep the 37 server `let _ =` swallows (the pattern
that silently hid migration-005's missing columns); dashboard renders the real
cause + correlation id (drop the hardcoded generic at MachinesPage.tsx:202);
agent logs why/where auth/connection failed (the auth-loop incident gave no
local signal). Phaseable; Large. Parallel RMM request keeps conventions aligned.
Requested by Mike 2026-05-30.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/FEATURE_ROADMAP.md

@@ -53,6 +53,7 @@ Bringing GC to parity with GuruRMM's release engineering. Full plan: [SPEC-001](
 - [ ] Machines "by Company" tree nav with per-company counts — P3 — left-nav grouping sidebar (screenshot parity). Follow-up sub-item of SPEC-005.
 - [ ] **Universal machine search ("everything is searchable")** — P2 — server-side `?q=` on `/api/machines` matching case-insensitive substring across ALL attributes (OS, logged-on user, external/private IP, company, site, tag, serial, MAC, version, …), pg_trgm GIN-indexed; multi-term AND + optional field-scoped syntax (`os:`, `user:`, `ip:`). Replaces the hostname-only client filter. Depends on SPEC-003 (attrs must be persisted). ([SPEC-006](specs/SPEC-006-universal-machine-search.md))
 - [ ] **Managed-agent installer builder ("Build Installer")** — P2 — dashboard wizard to build a pre-labeled persistent-agent installer (Name/Company/Site/Department/Device Type/Tag/Type) with Download / Copy URL / Send Link, reusing the existing embed-config download path; adds department + device_type to EmbeddedConfig/AgentStatus so labels persist at install time. Pairs with revocable per-machine keys; signature-vs-appended-config is the key open question. ([SPEC-007](specs/SPEC-007-managed-agent-installer-builder.md))
+- [ ] **Valuable error messages (structured errors + no silent swallows)** — P2 — one structured API error envelope with stable codes + a correlation id that also lands in the logs; contextual tracing on server/agent; sweep the 37 `let _ =` swallows (the pattern that hid the migration-005 bug); dashboard surfaces the real cause + id instead of a generic line. ([SPEC-008](specs/SPEC-008-valuable-error-messages.md))
 - [ ] Programmatic session pre-create + viewer-token (integration contract) — P2
 
 ## Security & Infrastructure