feat: greenfield Go rewrite (Phase 2A + 2B + 2C core)

Replaces the Python v1 (preserved on archive/python-v1 branch).

Stack: Go 1.23 + chi router + sqlx + MySQL 8. Distroless static
container. 12-factor config from env. Embedded SQL migrations.

Schema (internal/db/migrations/001_init.sql):
- topics: 议题 with 4-timestamp lifecycle (signup_open/close +
  debate_start/end), visibility (default private), status state machine,
  verdict_schema FK
- signups: agent self-enrollment with willing_camps (JSON array of
  pro|con|judge), pre_validated audit flag, (topic,agent) unique
- camps: post-allocation lock (one row per topic+camp) — written by
  Phase 2D allocator
- rounds + arguments: chronological debate transcript
- verdicts: judge structured output, one per topic, with token-cost
  trail for future budgeting
- agent_keys + system_keys: peppered sha256 hashes, never raw
- verdict_schemas: seeded with binary, claim-resolution (for
  analyze-intel), policy-recommendation, free-form

Auth (internal/auth):
- AgentAPIKey: real bearer-token middleware against agent_keys;
  best-effort last_used_at touch on success
- OIDCBrowser: Phase 2 stub. Dev mode accepts x-dev-bypass header
  (constant-time compare); prod 401s with a Phase-4-pending hint.
  Real Keycloak JWKS verification lands with the frontend rewrite.

HTTP API (internal/httpapi):
- /api/healthz — db ping + version + uptime
- GET /api/topics — list with status/visibility/limit/offset filters;
  anonymous callers see public only
- GET /api/topics/{id} — visibility-gated (private → 404 hide)
- POST /api/topics — create with RFC3339 lifecycle validation
  (signup_open < signup_close <= debate_start < debate_end)
- PUT /api/topics/{id}/visibility — dialectic-admin role gate
- POST /api/topics/{id}/signups — agent self-enroll; rejects when
  topic.status != signup_open OR outside signup window; idempotent
  upsert per (topic, agent)
- GET /api/topics/{id}/signups — list (any authed caller)

Auth chains:
- optionalAuth: try bearer → try oidc → fall through anonymous
  (handlers branch on Caller.Kind == ""). Uses captureWriter to demote
  inner 401s to "try next" without leaking response bytes.
- requireAnyAuth: chain that 401s if neither succeeds.
- requireAgent: strict bearer-only (signup POST).

Run: `docker compose -f docker-compose.dev.yml up --build`. Migrations
auto-apply on first connect; idempotent on reboot. README documents
env vars, dev bypass usage, agent-key provisioning SQL, and the
Phase 2D/E/3/4/5 roadmap.

go vet clean, gofmt clean, single 11M static binary.

internal/store/signup_store.go

@@ -0,0 +1,95 @@
+package store
+
+import (
+	"context"
+	"database/sql"
+	"encoding/json"
+	"errors"
+	"fmt"
+
+	"github.com/google/uuid"
+	"github.com/jmoiron/sqlx"
+
+	"git.hangman-lab.top/hzhang/Dialectic.Backend/internal/models"
+)
+
+type SignupStore struct {
+	db *sqlx.DB
+}
+
+func NewSignupStore(db *sqlx.DB) *SignupStore { return &SignupStore{db: db} }
+
+type UpsertSignupInput struct {
+	TopicID      string
+	AgentID      string
+	WillingCamps []models.Camp
+	PreValidated bool
+}
+
+// Upsert creates or updates an agent's signup for a topic. Re-signup
+// replaces willing_camps (intentional: lets an agent change their mind
+// before signup_close_at).
+func (s *SignupStore) Upsert(ctx context.Context, in UpsertSignupInput) (*models.SignupView, error) {
+	if len(in.WillingCamps) == 0 {
+		return nil, fmt.Errorf("willing_camps must be non-empty")
+	}
+	for _, c := range in.WillingCamps {
+		if !models.IsCampValid(c) {
+			return nil, fmt.Errorf("invalid camp %q", c)
+		}
+	}
+	raw, err := json.Marshal(in.WillingCamps)
+	if err != nil {
+		return nil, err
+	}
+
+	// Try insert; on duplicate (topic, agent), update.
+	id := uuid.NewString()
+	_, err = s.db.ExecContext(ctx, `
+		INSERT INTO signups (id, topic_id, agent_id, willing_camps, pre_validated)
+		VALUES (?, ?, ?, ?, ?)
+		ON DUPLICATE KEY UPDATE
+		  willing_camps = VALUES(willing_camps),
+		  pre_validated = VALUES(pre_validated)`,
+		id, in.TopicID, in.AgentID, raw, in.PreValidated)
+	if err != nil {
+		return nil, fmt.Errorf("upsert signup: %w", err)
+	}
+	return s.GetByPair(ctx, in.TopicID, in.AgentID)
+}
+
+func (s *SignupStore) GetByPair(ctx context.Context, topicID, agentID string) (*models.SignupView, error) {
+	var row models.Signup
+	err := s.db.GetContext(ctx, &row,
+		`SELECT * FROM signups WHERE topic_id = ? AND agent_id = ?`, topicID, agentID)
+	if errors.Is(err, sql.ErrNoRows) {
+		return nil, ErrNotFound
+	}
+	if err != nil {
+		return nil, err
+	}
+	v, err := row.View()
+	if err != nil {
+		return nil, err
+	}
+	return &v, nil
+}
+
+// ListByTopic returns all signups for a topic. Used by the allocation
+// algorithm at signup_close_at and by the topic-detail UI.
+func (s *SignupStore) ListByTopic(ctx context.Context, topicID string) ([]models.SignupView, error) {
+	var rows []models.Signup
+	if err := s.db.SelectContext(ctx, &rows,
+		`SELECT * FROM signups WHERE topic_id = ? ORDER BY created_at ASC`, topicID); err != nil {
+		return nil, err
+	}
+	out := make([]models.SignupView, 0, len(rows))
+	for _, r := range rows {
+		v, err := r.View()
+		if err != nil {
+			return nil, err
+		}
+		out = append(out, v)
+	}
+	return out, nil
+}