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.

Dockerfile

@@ -1,22 +1,19 @@
-FROM python:3.13-slim
+# syntax=docker/dockerfile:1.7
+# Dialectic.Backend.Go — multi-stage build (compile static binary, run on distroless).
 
-ENV PYTHONDONTWRITEBYTECODE=1
-ENV PYTHONUNBUFFERED=1
+FROM golang:1.23-bookworm AS build
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+ARG VERSION=dev
+RUN CGO_ENABLED=0 GOOS=linux go build \
+    -ldflags="-s -w -X main.Version=${VERSION}" \
+    -o /out/dialectic-backend .
 
+FROM gcr.io/distroless/static-debian12:nonroot
 WORKDIR /app
-
-RUN apt-get update && apt-get install -y --no-install-recommends \
-    build-essential gcc \
-    python3-dev \
-    libssl-dev libffi-dev \
-    && rm -rf /var/lib/apt/lists/*
-
-RUN python -m pip install --upgrade pip setuptools wheel
-
-COPY requirements.txt ./requirements.txt
-RUN pip install --no-cache-dir -r requirements.txt
-
-COPY . /app/
-
-EXPOSE 8000
-CMD ["python3", "app.py"]
+COPY --from=build /out/dialectic-backend /app/dialectic-backend
+EXPOSE 8090
+USER nonroot:nonroot
+ENTRYPOINT ["/app/dialectic-backend"]