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).

migrations/021_solver_results.sql

@@ -0,0 +1,29 @@
+-- Migration 021: Solver result cache table
+--
+-- Stores the latest solve/diagnose/kinematic result per item revision.
+-- The UNIQUE constraint means re-running an operation overwrites the previous result.
+-- See docs/SOLVER.md Section 9.
+
+BEGIN;
+
+CREATE TABLE solver_results (
+    id              UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    item_id         UUID NOT NULL REFERENCES items(id) ON DELETE CASCADE,
+    revision_number INTEGER NOT NULL,
+    job_id          UUID REFERENCES jobs(id) ON DELETE SET NULL,
+    operation       TEXT NOT NULL,         -- 'solve', 'diagnose', 'kinematic'
+    solver_name     TEXT NOT NULL,
+    status          TEXT NOT NULL,         -- SolveStatus string ('Success', 'Failed', etc.)
+    dof             INTEGER,
+    diagnostics     JSONB DEFAULT '[]',
+    placements      JSONB DEFAULT '[]',
+    num_frames      INTEGER DEFAULT 0,
+    solve_time_ms   DOUBLE PRECISION,
+    created_at      TIMESTAMPTZ NOT NULL DEFAULT now(),
+    UNIQUE(item_id, revision_number, operation)
+);
+
+CREATE INDEX idx_solver_results_item ON solver_results(item_id);
+CREATE INDEX idx_solver_results_status ON solver_results(status);
+
+COMMIT;