feat(auth): add API-key authentication and tenant resolution

src/application/auth/__init__.py

@@ -0,0 +1,29 @@
+"""API-key authentication and tenant resolution (ADR-0008).
+
+`resolve_auth_context` is the entry point: it takes a bearer token and
+returns a trusted `AuthContext`. Everything downstream of the FastAPI
+boundary receives `tenant_id` only through that context — never from a
+request body, query string, or object metadata.
+"""
+
+from src.application.auth.context import AuthContext
+from src.application.auth.errors import (
+    AuthError,
+    InvalidApiKeyError,
+    MissingScopeError,
+    TenantInactiveError,
+)
+from src.application.auth.keys import generate_api_key, hash_secret, verify_secret
+from src.application.auth.service import resolve_auth_context
+
+__all__ = [
+    "AuthContext",
+    "AuthError",
+    "InvalidApiKeyError",
+    "MissingScopeError",
+    "TenantInactiveError",
+    "generate_api_key",
+    "hash_secret",
+    "resolve_auth_context",
+    "verify_secret",
+]

src/application/auth/context.py

@@ -0,0 +1,16 @@
+"""The trusted request-scoped auth/tenant context (ADR-0008)."""
+
+import uuid
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class AuthContext:
+    tenant_id: uuid.UUID
+    tenant_slug: str
+    api_key_id: uuid.UUID
+    scopes: frozenset[str]
+    actor_type: str
+
+    def has_scope(self, scope: str) -> bool:
+        return scope in self.scopes or "admin" in self.scopes

src/application/auth/errors.py

@@ -0,0 +1,22 @@
+"""Auth failures (ADR-0008). No HTTP knowledge here — `src/api/errors.py` maps
+these to status codes.
+"""
+
+
+class AuthError(Exception):
+    """Base class for auth failures."""
+
+
+class InvalidApiKeyError(AuthError):
+    """The bearer token is missing, malformed, unknown, revoked, or expired.
+
+    Maps to `401`.
+    """
+
+
+class TenantInactiveError(AuthError):
+    """The key's tenant is suspended or deleted. Maps to `401`."""
+
+
+class MissingScopeError(AuthError):
+    """The key is valid but lacks a scope the route requires. Maps to `403`."""

src/application/auth/keys.py

@@ -0,0 +1,39 @@
+"""API-key generation and hashing (ADR-0008, ADR-0009).
+
+Keys are `sk_{prefix}_{secret}`. `prefix` is non-secret and indexed
+(`api_keys.key_prefix`); `secret` is 256 bits of `secrets.token_urlsafe`
+entropy, stored only as a SHA-256 hash. A random 256-bit secret does not
+benefit from a slow password-hashing KDF the way a human-chosen password
+does — the cost that defends against dictionary/brute-force guessing over a
+low-entropy input has nothing to defend here, and would only tax every
+request. Comparison is constant-time to avoid a hash-timing oracle.
+"""
+
+import hashlib
+import hmac
+import secrets
+
+_PREFIX_LENGTH = 16
+
+
+def generate_api_key() -> tuple[str, str, str]:
+    """Return `(key_prefix, secret, full_key)` for a newly issued key."""
+    key_prefix = secrets.token_hex(_PREFIX_LENGTH // 2)
+    secret = secrets.token_urlsafe(32)
+    return key_prefix, secret, f"sk_{key_prefix}_{secret}"
+
+
+def parse_api_key(full_key: str) -> tuple[str, str] | None:
+    """Return `(key_prefix, secret)`, or `None` if the token is malformed."""
+    parts = full_key.split("_", 2)
+    if len(parts) != 3 or parts[0] != "sk" or not parts[1] or not parts[2]:
+        return None
+    return parts[1], parts[2]
+
+
+def hash_secret(secret: str) -> str:
+    return hashlib.sha256(secret.encode("utf-8")).hexdigest()
+
+
+def verify_secret(secret: str, key_hash: str) -> bool:
+    return hmac.compare_digest(hash_secret(secret), key_hash)

src/application/auth/service.py

@@ -0,0 +1,47 @@
+"""Resolve a bearer token to a trusted `AuthContext` (ADR-0008).
+
+This opens and releases its own session rather than borrowing a
+request-scoped one, so auth resolution never pins a pool connection across
+the rest of the request — including the ADR-0017 ingestion work phase, which
+must run with no Postgres session held open at all.
+"""
+
+from datetime import UTC, datetime
+
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker
+
+from src.application.auth.context import AuthContext
+from src.application.auth.errors import InvalidApiKeyError, TenantInactiveError
+from src.application.auth.keys import parse_api_key, verify_secret
+from src.infrastructure.postgres.repositories import api_keys as api_keys_repo
+from src.infrastructure.postgres.repositories import tenants as tenants_repo
+
+
+async def resolve_auth_context(
+    sessionmaker: async_sessionmaker[AsyncSession], bearer_token: str
+) -> AuthContext:
+    parsed = parse_api_key(bearer_token)
+    if parsed is None:
+        raise InvalidApiKeyError("malformed API key")
+    key_prefix, secret = parsed
+
+    async with sessionmaker() as session:
+        api_key = await api_keys_repo.get_by_prefix(session, key_prefix)
+        if api_key is None or not verify_secret(secret, api_key.key_hash):
+            raise InvalidApiKeyError("unknown API key")
+        if api_key.status != "active":
+            raise InvalidApiKeyError(f"API key is {api_key.status}")
+        if api_key.expires_at is not None and api_key.expires_at <= datetime.now(UTC):
+            raise InvalidApiKeyError("API key has expired")
+
+        tenant = await tenants_repo.get_by_id(session, api_key.tenant_id)
+        if tenant is None or tenant.status != "active":
+            raise TenantInactiveError("tenant is not active")
+
+        return AuthContext(
+            tenant_id=tenant.id,
+            tenant_slug=tenant.slug,
+            api_key_id=api_key.id,
+            scopes=frozenset(api_key.scopes),
+            actor_type=api_key.actor_type,
+        )