docs(adr): record modular monolith decision as ADR-001

Adds docs/adr/ADR-001-modular-monolith.md with the six required sections
(Context, Decision, Alternatives, Consequences, Operational impact, Revisit
trigger). The decision text ("Modular monolith") matches the ADR index entry
in section 70. Documentation-only; no runtime or schema impact.

docs/adr/ADR-001-modular-monolith.md

@@ -0,0 +1,107 @@
+# ADR-001: Modular monolith
+
+- Status: Accepted
+- Date: 2026-08-31
+- Deciders: platform stream
+- References: ADR index (section 70), Architecture wiki (sections 1, 4, 9)
+
+## Context
+
+EPPP is a greenfield personal blogging platform. The first visible release
+(v0.1) is intentionally small: editable site identity, editable Home page,
+ordered list of published posts, individual post pages, an administration
+interface and the Amber theme. It must be small because optional features are
+absent, not because extensibility is absent — the codebase ships explicit,
+versioned boundaries from day one for content types, content blocks, page
+sections, themes, extensions, extension settings, navigation, visitor
+preferences, database migrations, media storage, background jobs, events and
+rendering.
+
+The platform is owned by a single small stream, operates a single canonical
+PostgreSQL database, and has no v1 requirement for independent service
+deployment, service discovery, message brokers, distributed transactions or
+cross-service schema versioning. The workspace is a pnpm monorepo whose
+dependency boundaries are already CI-enforced (FIT-001, FIT-010, FIT-011), so
+module boundaries can be structural, explicit and testable rather than purely
+organisational.
+
+## Decision
+
+EPPP is a **modular monolith**. One application image contains the public
+server, admin API, rendering pipeline, extension runtime, core services, built
+admin assets and first-party extensions, deployed as a single deployable unit;
+an optional worker uses the same image with a different command (`eppp serve`
+/ `eppp worker`). Runtime is Node.js 24 LTS with Fastify 5, PostgreSQL 18 as
+the sole canonical database, React 19 for server-rendered public components
+and a Vite admin, and Docker Compose as the primary runtime model.
+
+Modules are separated in the workspace layout (`apps/`, `packages/`,
+`extensions/`) and their import edges are enforced by
+`dependency-boundaries.json` in CI. Every module boundary is an explicit,
+versioned contract; no module may reach into another module's internals. The
+decision text — **Modular monolith** — matches the ADR index entry (ADR-001,
+section 70).
+
+## Alternatives
+
+- **Microservices / distributed system** — rejected: no v1 requirement
+  justifies distributed transactions, service discovery, brokers,
+  multi-service deployment or cross-service schema changes; the operational
+  and cognitive cost is disproportionate for a small platform stream, and it
+  would split one canonical database into many.
+- **Classic (unstructured) monolith** — rejected: it would undermine the
+  extension architecture, which depends on stable versioned contracts between
+  core, extensions and themes, and it would make the CI-enforced dependency
+  boundaries impossible to honour in practice.
+- **Serverless / FaaS** — rejected: it conflicts with the chosen long-lived
+  runtime model (Fastify 5 process, background jobs, connection-backed
+  PostgreSQL access) and the single-image deployment model.
+- **Modular monolith with future extraction** — chosen: module boundaries
+  exist now; any module can be extracted into a service later on evidence
+  without a rewrite of the whole system.
+
+## Consequences
+
+- Positive: one deployable unit means simpler operations, atomic deploys and a
+  single image to build, scan and promote; all modules share one transaction
+  and consistency boundary via the canonical database; a small team can own
+  the entire platform; explicit boundaries force API discipline and keep the
+  extension contracts honest.
+- Negative: the single process only stays modular through discipline — a
+  boundary violation degrades it toward a big ball of mud, which is exactly
+  what the CI dependency-boundary tests guard against; scaling is vertical
+  until a module is extracted; deploys are all-or-nothing; a fault in one
+  module can affect the whole process (mitigated by the optional worker
+  separation for background jobs and by health/readiness gates).
+- Neutral: extraction of a module into a service remains possible and is a
+  deliberate, evidence-based decision rather than a default; the monorepo
+  layout already supports it because modules are physically separated.
+
+## Operational impact
+
+- One application image/container for the server plus an optional worker
+  container running the same image with a different command; no service
+  discovery, message broker or per-service observability requirements at v1.
+- Single PostgreSQL 18 instance as the sole canonical database with one core
+  migration chain; extension-owned tables live in independent chains
+  (`eppp_extension_migrations`), all run behind the advisory migration lock at
+  startup.
+- The health/readiness endpoint gates on migration completion; readiness is
+  reported only when the single deployable unit is fully initialised.
+- Deploy is build-one-image-and-run-Compose; rollback is redeploying the
+  previous image. Horizontal scaling means running more instances of the same
+  image behind a proxy for stateless work and more workers for background
+  jobs; the database remains the single shared store.
+
+## Revisit trigger
+
+- Revisit this ADR when a module's change frequency, team ownership or scaling
+  needs diverge enough that a single deployable unit becomes a bottleneck —
+  for example, when a module needs an independent deploy cadence, independent
+  scaling or a different runtime — and there is evidence (per the architecture
+  review gates, ADR index section 68) that extracting that module into a
+  service would help.
+- Revisit if the platform grows to multiple independent products or teams
+  requiring distributed transactions or independent data stores, or if the
+  v1.1 boundary contract (ADR-027 to ADR-032) pressures the single-image
+  model.