Add Phase 1 foundation: Go module, core types, DB infrastructure, config

Establish the project foundation with three packages:
- core: shared types (Header, Metadata, Value, ObjectType, UUID generation)
- db: SQLite migration framework, connection management (WAL, FK, busy
  timeout), transaction helpers (StartTX/EndTX), time conversion
- config: runtime configuration (DB path, blob store, Minio, gRPC addr)

Includes initial schema migration (001_initial.sql) with 13 tables covering
shared infrastructure, bibliographic data, and artifact repository. Full test
coverage for all packages, strict linting (.golangci.yaml), and Makefile.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

db/migrations/001_initial.sql

@@ -0,0 +1,128 @@
+-- Migration 001: Initial schema
+-- Shared infrastructure, bibliographic tables, and artifact repository.
+
+-- Polymorphic key-value metadata. The id column references any object's UUID.
+CREATE TABLE IF NOT EXISTS metadata
+(
+    id       TEXT NOT NULL,
+    mkey     TEXT NOT NULL,
+    contents TEXT NOT NULL,
+    type     TEXT NOT NULL,
+    PRIMARY KEY (mkey, contents, type),
+    UNIQUE (id, mkey)
+);
+CREATE INDEX IF NOT EXISTS idx_metadata_id ON metadata (id);
+
+-- Shared tag pool (used by both artifacts and knowledge graph nodes).
+CREATE TABLE IF NOT EXISTS tags
+(
+    id  TEXT NOT NULL PRIMARY KEY,
+    tag TEXT NOT NULL UNIQUE
+);
+
+-- Shared category pool.
+CREATE TABLE IF NOT EXISTS categories
+(
+    id       TEXT NOT NULL PRIMARY KEY,
+    category TEXT NOT NULL UNIQUE
+);
+
+-- Publishers for bibliographic citations.
+CREATE TABLE IF NOT EXISTS publishers
+(
+    id      TEXT UNIQUE NOT NULL PRIMARY KEY,
+    name    TEXT        NOT NULL,
+    address TEXT,
+    UNIQUE (name, address)
+);
+
+-- Bibliographic citations.
+CREATE TABLE IF NOT EXISTS citations
+(
+    id        TEXT PRIMARY KEY,
+    doi       TEXT,
+    title     TEXT    NOT NULL,
+    year      INTEGER NOT NULL,
+    published TEXT    NOT NULL,
+    publisher TEXT    NOT NULL,
+    source    TEXT    NOT NULL,
+    abstract  TEXT,
+    FOREIGN KEY (publisher) REFERENCES publishers (id)
+);
+CREATE INDEX IF NOT EXISTS idx_citations_doi ON citations (id, doi);
+
+-- Many-to-one: multiple authors per citation.
+CREATE TABLE IF NOT EXISTS authors
+(
+    citation_id TEXT NOT NULL,
+    author_name TEXT NOT NULL,
+    FOREIGN KEY (citation_id) REFERENCES citations (id)
+);
+
+-- Artifact repository.
+CREATE TABLE IF NOT EXISTS artifacts
+(
+    id          TEXT PRIMARY KEY,
+    type        TEXT NOT NULL,
+    citation_id TEXT NOT NULL,
+    latest      TEXT NOT NULL,
+    FOREIGN KEY (citation_id) REFERENCES citations (id)
+);
+
+-- Many-to-many junction: artifacts <-> tags.
+CREATE TABLE IF NOT EXISTS artifact_tags
+(
+    artifact_id TEXT NOT NULL,
+    tag_id      TEXT NOT NULL,
+    FOREIGN KEY (artifact_id) REFERENCES artifacts (id),
+    FOREIGN KEY (tag_id) REFERENCES tags (id)
+);
+
+-- Many-to-many junction: artifacts <-> categories.
+CREATE TABLE IF NOT EXISTS artifact_categories
+(
+    artifact_id TEXT NOT NULL,
+    category_id TEXT NOT NULL,
+    FOREIGN KEY (artifact_id) REFERENCES artifacts (id),
+    FOREIGN KEY (category_id) REFERENCES categories (id)
+);
+
+-- Temporal index linking artifacts to snapshots by datetime.
+CREATE TABLE IF NOT EXISTS artifacts_history
+(
+    artifact_id TEXT NOT NULL,
+    snapshot_id TEXT NOT NULL UNIQUE,
+    datetime    TEXT NOT NULL,
+    PRIMARY KEY (artifact_id, datetime),
+    FOREIGN KEY (artifact_id) REFERENCES artifacts (id)
+);
+
+-- Snapshot records with storage and content timestamps.
+CREATE TABLE IF NOT EXISTS artifact_snapshots
+(
+    artifact_id TEXT    NOT NULL,
+    id          TEXT UNIQUE PRIMARY KEY,
+    stored_at   INTEGER NOT NULL,
+    datetime    TEXT    NOT NULL,
+    citation_id TEXT    NOT NULL,
+    source      TEXT    NOT NULL,
+    FOREIGN KEY (artifact_id) REFERENCES artifacts (id),
+    FOREIGN KEY (id) REFERENCES artifacts_history (snapshot_id)
+);
+
+-- Blob registry. Actual content lives in the CAS on disk.
+CREATE TABLE IF NOT EXISTS blobs
+(
+    snapshot_id TEXT NOT NULL,
+    id          TEXT NOT NULL UNIQUE PRIMARY KEY,
+    format      TEXT NOT NULL,
+    FOREIGN KEY (snapshot_id) REFERENCES artifact_snapshots (id)
+);
+
+-- Schema version tracking.
+CREATE TABLE IF NOT EXISTS schema_version
+(
+    version INTEGER NOT NULL,
+    applied TEXT    NOT NULL
+);
+INSERT INTO schema_version (version, applied) VALUES (1, datetime('now'));