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/config/config.go

@@ -19,6 +19,7 @@ type Config struct {
 	Auth      AuthConfig      `yaml:"auth"`
 	Jobs      JobsConfig      `yaml:"jobs"`
 	Workflows WorkflowsConfig `yaml:"workflows"`
+	Solver    SolverConfig    `yaml:"solver"`
 	Modules   ModulesConfig   `yaml:"modules"`
 }
 
@@ -32,6 +33,7 @@ type ModulesConfig struct {
 	FreeCAD  *ModuleToggle `yaml:"freecad"`
 	Jobs     *ModuleToggle `yaml:"jobs"`
 	DAG      *ModuleToggle `yaml:"dag"`
+	Solver   *ModuleToggle `yaml:"solver"`
 }
 
 // ModuleToggle holds an optional enabled flag. The pointer allows
@@ -146,6 +148,14 @@ type WorkflowsConfig struct {
 	Directory string `yaml:"directory"` // default /etc/silo/workflows
 }
 
+// SolverConfig holds assembly solver service settings.
+type SolverConfig struct {
+	DefaultSolver        string `yaml:"default_solver"`
+	MaxContextSizeMB     int    `yaml:"max_context_size_mb"`
+	DefaultTimeout       int    `yaml:"default_timeout"`
+	AutoDiagnoseOnCommit bool   `yaml:"auto_diagnose_on_commit"`
+}
+
 // OdooConfig holds Odoo ERP integration settings.
 type OdooConfig struct {
 	Enabled  bool   `yaml:"enabled"`
@@ -204,6 +214,12 @@ func Load(path string) (*Config, error) {
 	if cfg.Workflows.Directory == "" {
 		cfg.Workflows.Directory = "/etc/silo/workflows"
 	}
+	if cfg.Solver.MaxContextSizeMB == 0 {
+		cfg.Solver.MaxContextSizeMB = 10
+	}
+	if cfg.Solver.DefaultTimeout == 0 {
+		cfg.Solver.DefaultTimeout = 300
+	}
 
 	// Override with environment variables
 	if v := os.Getenv("SILO_DB_HOST"); v != "" {
@@ -221,6 +237,9 @@ func Load(path string) (*Config, error) {
 	if v := os.Getenv("SILO_STORAGE_ROOT_DIR"); v != "" {
 		cfg.Storage.Filesystem.RootDir = v
 	}
+	if v := os.Getenv("SILO_SOLVER_DEFAULT"); v != "" {
+		cfg.Solver.DefaultSolver = v
+	}
 
 	// Auth defaults
 	if cfg.Auth.LDAP.UserAttr == "" {