feat(api): solver service Phase 3b — server endpoints, job definitions, and result cache

Add server-side solver service module with REST API endpoints, database
schema, job definitions, and runner result caching.

New files:
- migrations/021_solver_results.sql: solver_results table with upsert constraint
- internal/db/solver_results.go: SolverResultRepository (Upsert, GetByItem, GetByItemRevision)
- internal/api/solver_handlers.go: solver API handlers and maybeCacheSolverResult hook
- jobdefs/assembly-solve.yaml: manual solve job definition
- jobdefs/assembly-validate.yaml: auto-validate on revision creation
- jobdefs/assembly-kinematic.yaml: manual kinematic simulation job

Modified:
- internal/config/config.go: SolverConfig struct with max_context_size_mb, default_timeout
- internal/modules/modules.go, loader.go: register solver module (depends on jobs)
- internal/db/jobs.go: ListSolverJobs helper with definition_name prefix filter
- internal/api/handlers.go: wire SolverResultRepository into Server
- internal/api/routes.go: /api/solver/* routes + /api/items/{partNumber}/solver/results
- internal/api/runner_handlers.go: async result cache hook on job completion

API endpoints:
- POST   /api/solver/jobs          — submit solver job (editor)
- GET    /api/solver/jobs          — list solver jobs with filters
- GET    /api/solver/jobs/{id}     — get solver job status
- POST   /api/solver/jobs/{id}/cancel — cancel solver job (editor)
- GET    /api/solver/solvers       — registry of available solvers
- GET    /api/items/{pn}/solver/results — cached results for item

Also fixes pre-existing test compilation errors (missing workflows param
in NewServer calls across 6 test files).

internal/modules/modules.go

@@ -21,6 +21,7 @@ const (
 	FreeCAD  = "freecad"
 	Jobs     = "jobs"
 	DAG      = "dag"
+	Solver   = "solver"
 )
 
 // ModuleInfo describes a module's metadata.
@@ -58,6 +59,7 @@ var builtinModules = []ModuleInfo{
 	{ID: FreeCAD, Name: "Create Integration", Description: "URI scheme, executable path, client settings", DefaultEnabled: true},
 	{ID: Jobs, Name: "Job Queue", Description: "Async compute jobs, runner management", DependsOn: []string{Auth}},
 	{ID: DAG, Name: "Dependency DAG", Description: "Feature DAG sync, validation states, interference detection", DependsOn: []string{Jobs}},
+	{ID: Solver, Name: "Solver", Description: "Assembly constraint solving via server-side runners", DependsOn: []string{Jobs}},
 }
 
 // NewRegistry creates a registry with all builtin modules set to their default state.